spi: renesas,sh-msiof: Add r8a77961 support

Document R-Car M3-W+ (R8A77961) SoC bindings.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20201005112549.22222-1-geert+renesas@glider.be


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