dt: snps,dw-apb-ssi: Document new I/O data register width property

This change documents a new property for the snps,dw-apb-ssi device,
allowing an implementer to specify either four byte or two bytes
access to the SPI controller data register.

This supports a change that unbreaks this driver on picoXcell
platforms.
Signed-off-by: Michael van der Westhuizen <michael@smart-africa.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/snps,dw-apb-ssi.txt

@@ -10,6 +10,8 @@ Required properties:
 Optional properties:
 - cs-gpios : Specifies the gpio pis to be used for chipselects.
 - num-cs : The number of chipselects. If omitted, this will default to 4.
+- reg-io-width : The I/O register width (in bytes) implemented by this
+  device.  Supported values are 2 or 4 (the default).
 
 Child nodes as per the generic SPI binding.