spi: sh-msiof: Document R-Car D3 support

Document support for the MSIOF module in the Renesas R-Car D3 (r8a77995)
SoC.

No driver update is needed.

Signed-off-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>