ARM: dts: r8a7778: Use SoC-specific compat string for mmcif

Use the SoC-specific compat string for mmcif in DT for the r8a7778 SoC.
This is in keeping with the use of compat strings for mmcif for other
Renesas ARM based SoCs.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>