pinctrl: sh-pfc: r8a7791: Add SCIF_CLK support

Add pins, groups, and a function for SCIF_CLK, which is the external
clock source for the Baud Rate Generator for External Clock (BRG) on
(H)SCIF.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Linus Walleij <linus.walleij@linaro.org>