spi: rspi: Abstract 8/16-bit Data Register access

Add rspi_{write,read}_data(), to abstract 8-bit (QSPI, and RSPI on RZ/A1H)
versus 16-bit (RSPI) Data Register access.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Signed-off-by: Mark Brown <broonie@linaro.org>