mfd: syscon: Add Atmel SMC binding doc

The SMC registers are used to configure Atmel EBI (External Bus Interface)
to interface with standard memory devices (NAND, NOR, SRAM or specialized
devices like FPGAs).

Declare this memory region as a syscon, so that different drivers can
configure the SMC interface (mostly timing configuration) according to
their need.

Signed-off-by: Boris Brezillon <[e-mail]>
Acked-by: Lee Jones <[e-mail]>
Signed-off-by: Nicolas Ferre <[e-mail]>

Documentation/devicetree/bindings/mfd/atmel-smc.txt

···
+* Device tree bindings for Atmel SMC (Static Memory Controller)
+
+The SMC registers are used to configure Atmel EBI (External Bus Interface)
+to interface with standard memory devices (NAND, NOR, SRAM or specialized
+devices like FPGAs).
+
+Required properties:
+- compatible:		Should be one of the following
+			"atmel,at91sam9260-smc", "syscon"
+			"atmel,sama5d3-smc", "syscon"
+- reg:			Contains offset/length value of the SMC memory
+			region.
+
+Example:
+
+smc: smc@ffffc000 {
+	compatible = "atmel,sama5d3-smc", "syscon";
+	reg = <0xffffc000 0x1000>;
+};