dt-bindings: memory: document Renesas RPC-IF bindings

dt-bindings: memory: document Renesas RPC-IF bindings

Renesas Reduced Pin Count Interface (RPC-IF) allows a SPI flash or
HyperFlash connected to the SoC to be accessed via the external address
space read mode or the manual mode.

Document the device tree bindings for the Renesas RPC-IF found in the R-Car
gen3 SoCs.

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/54a84c75-fa17-9976-d9a6-a69ef67c418b@cogentembedded.com
Signed-off-by: Mark Brown <broonie@kernel.org>