dt-bindings: mmc: renesas_sdhi: Add r8a77470 support

The RZ/G1C (a.k.a. R8A77470) comes with three SDHI interfaces,
SDHI0 and SDHI2 are compatible with R-Car Gen2 SDHIs, and
SDHI1 is compatible with R-Car Gen3 SDHIs, as it comes with an
internal DMAC, therefore SDHI1 is fully compatible with driver
renesas_sdhi_internal_dmac driver. As a result, the compatible
strings for the R8A77470 SDHI interfaces are a little bit special.
Document SDHI support for the RZ/G1C SoC.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>