dt-binding: memory: pl353-smc: Document the range property

The ranges property is missing in the description while actually used in
the example. This property is actually needed, so mention it.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210610082040.2075611-3-miquel.raynal@bootlin.comSigned-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>

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

@@ -11,6 +11,8 @@ Required properties:
 - clocks		: Clock phandles (see clock bindings for details).
 - address-cells		: Must be 2.
 - size-cells		: Must be 1.
+- ranges		: Memory bus areas for interacting with the devices.
+			  Encodes CS to memory region association.
 
 The child device node represents the controller connected to the SMC
 bus. Only one between: NAND controller, NOR controller and SRAM controller