spi: rspi: Add rspi_data_{out,in,out_in}() helpers

Add helpers rspi_data_{out,in,out_in}() to write, read, or write and
read data to/from the Data Register, taking care of waiting until data
or space is available in the buffers.

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