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>