pinctrl: sh-pfc: r8a7791: Add missing mmc_data8_b pin group

Pins D6 and D7 of the MMC interface can be muxed to two different sets
of pins, but currently only one set is supported.
Add a pin group for the alternative set to fix this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Chris Paterson <chris.paterson2@renesas.com>
Tested-by: Chris Paterson <chris.paterson2@renesas.com>