spi: sh-msiof: document R8A779{7|8}0 bindings

Document the R-Car V3{M|H} (R8A779{7|8}0) SoCs in the Renesas MSIOF
bindings.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Mark Brown <broonie@kernel.org>