mmc: sh_mmcif: Document r8a73a4, r8a7778 and sh73a0 DT bindings

Simply document new compatibility strings as the driver is already
activated using a fallback compatibility string.

These compat strings are in keeping with those for all other
Renesas ARM based SoCs with sh_mmcif enabled in mainline.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>