spi: sh-msiof: Document R-Car E3 support

Document support for the MSIOF module in the Renesas R-Car E3 (r8a77990)
SoC.

No driver update is needed.

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