spi: add Renesas RPC-IF driver

Add the SPI driver for the Renesas RPC-IF.  It's the "front end" driver
using the "back end" APIs in the main driver to talk to the real hardware.
We only implement the 'spi-mem' interface -- there's no need to implement
the usual SPI driver methods...

Based on the original patch by Mason Yang <masonccyang@mxic.com.tw>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Link: https://lore.kernel.org/r/1ece0e6c-71af-f0f1-709e-571f4b0b4853@cogentembedded.com


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