dt-bindings: mmc: tmio_mmc: Document Renesas R7S9210

Document support for the RZ/A2 (R7S9210) SoC.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>