Documentation: dt-bindings: minimal documentation for MVEBU SDRAM controller

The suspend/resume code for Armada XP has to modify certain registers
of the SDRAM controller. Therefore, we need to define a Device Tree
binding for this hardware block.

Signed-off-by: Thomas Petazzoni <[e-mail]>
Cc: [e-mail]
Cc: Kumar Gala <[e-mail]>
Cc: Ian Campbell <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Rob Herring <[e-mail]>
Reviewed-by: Gregory CLEMENT <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Jason Cooper <[e-mail]>

Documentation/devicetree/bindings/memory-controllers/mvebu-sdram-controller.txt

···
+Device Tree bindings for MVEBU SDRAM controllers
+
+The Marvell EBU SoCs all have a SDRAM controller. The SDRAM controller
+differs from one SoC variant to another, but they also share a number
+of commonalities.
+
+For now, this Device Tree binding documentation only documents the
+Armada XP SDRAM controller.
+
+Required properties:
+
+ - compatible: for Armada XP, "marvell,armada-xp-sdram-controller"
+ - reg: a resource specifier for the register space, which should
+   include all SDRAM controller registers as per the datasheet.
+
+Example:
+
+sdramc@1400 {
+	compatible = "marvell,armada-xp-sdram-controller";
+	reg = <0x1400 0x500>;
+};