dt-bindings: mtd: make partitions doc a bit more generic

Currently the only documented partitioning is "fixed-partitions" but
there are more methods in use that we may want to support in the future.
Mention them and make it clear Fixed Partitions are just a single case.

Signed-off-by: Brian Norris <[e-mail]>
Signed-off-by: Rafał Miłecki <[e-mail]>
Acked-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/mtd/partition.txt

···
-Representing flash partitions in devicetree
+Flash partitions in device tree
+===============================
 
-Partitions can be represented by sub-nodes of an mtd device. This can be used
+Flash devices can be partitioned into one or more functional ranges (e.g. "boot
+code", "nvram", "kernel").
+
+Different devices may be partitioned in a different ways. Some may use a fixed
+flash layout set at production time. Some may use on-flash table that describes
+the geometry and naming/purpose of each functional region. It is also possible
+to see these methods mixed.
+
+To assist system software in locating partitions, we allow describing which
+method is used for a given flash device. To describe the method there should be
+a subnode of the flash device that is named 'partitions'. It must have a
+'compatible' property, which is used to identify the method to use.
+
+We currently only document a binding for fixed layouts.
+
+
+Fixed Partitions
+================
+
+Partitions can be represented by sub-nodes of a flash device. This can be used
 on platforms which have strong conventions about which portions of a flash are
 used for what purposes, but which don't use an on-flash partition table such
 as RedBoot.
 
-The partition table should be a subnode of the mtd node and should be named
+The partition table should be a subnode of the flash node and should be named
 'partitions'. This node should have the following property:
 - compatible : (required) must be "fixed-partitions"
 Partitions are then defined in subnodes of the partitions node.
 
-For backwards compatibility partitions as direct subnodes of the mtd device are
+For backwards compatibility partitions as direct subnodes of the flash device are
 supported. This use is discouraged.
 NOTE: also for backwards compatibility, direct subnodes that have a compatible
 string are not considered partitions, as they may be used for other bindings.
 
 #address-cells & #size-cells must both be present in the partitions subnode of the
-mtd device. There are two valid values for both:
+flash device. There are two valid values for both:
 <1>: for partitions that require a single 32-bit cell to represent their
      size/address (aka the value is below 4 GiB)
 <2>: for partitions that require two 32-bit cells to represent their
      size/address (aka the value is 4 GiB or greater).
 
 Required properties:
-- reg : The partition's offset and size within the mtd bank.
+- reg : The partition's offset and size within the flash
 
 Optional properties:
 - label : The label / name for this partition.  If omitted, the label is taken