pinctrl: renesas: r8a7791: Share MMC pin group data

Pin groups mmc_data[14] are subsets of mmc_data8.

Treat mmc_data8_b the same for consistency.  Note that there is no need
to define 1-bit and 4-bit wide subsets for the b-variant, as they're
identical to the normal variants.

This reduces kernel size by 40 bytes.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/be1f11c87e51224e043291f4d8e28b620811ac76.1640269757.git.geert+renesas@glider.be