dt-bindings: spi: sh-msiof: Add r8a774b1 support

Document RZ/G2N (R8A774B1) SoC bindings.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/1570178133-21532-3-git-send-email-fabrizio.castro@bp.renesas.com


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