pinctrl: sh-pfc: r8a7795: Add SCIF_CLK support

Add pins, groups, and a function for SCIF_CLK on R-Car H3 ES2.0.
SCIF_CLK is the external clock source for the Baud Rate Generator for
External Clock (BRG) on (H)SCIF serial ports.

Extracted from a big patch in the BSP by Takeshi Kihara.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Takeshi Kihara <takeshi.kihara.df@renesas.com>