spi: rspi: Add support for LSB-first word order

All RSPI variants support selecting the word order.
Advertize support for LSB-first order, and act upon the flag being set.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200218105810.902-3-geert+renesas@glider.be


Signed-off-by: Mark Brown <broonie@kernel.org>