dt-bindings: mmc: sh_mmcif: Document r8a7744 DT bindings

Add support for r8a7744 SoC. Renesas RZ/G1N (R8A7744) MMCIF is identical
to the R-Car Gen2 family.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>