dt-binding: memory: pl353-smc: Describe the child reg property

Each chil node should have a reg property, no matter the type of
controller (NAND, NOR, SRAM). This should be part of the bindings.
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210610082040.2075611-5-miquel.raynal@bootlin.comSigned-off-by: NKrzysztof Kozlowski <krzysztof.kozlowski@canonical.com>

Documentation/devicetree/bindings/memory-controllers/pl353-smc.txt

@@ -18,6 +18,10 @@ The child device node represents the controller connected to the SMC
 bus. Only one between: NAND controller, NOR controller and SRAM controller
 is allowed in a single system.
 
+Required device node properties:
+
+- reg:			Contains the chip-select id, the offset and the length
+			of the memory region requested by the device.
 
 Example:
 	smcc: memory-controller@e000e000