spi: rspi: Extract rspi_pio_transfer()

The various PIO loops are very similar. Consolidate into a single
function rspi_pio_transfer().

Both buffer pointers can be NULL, as RSPI supports TX-only mode, and
Dual/Quad SPI Transfers are unidirectional.

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