pinctrl: sh-pfc: r8a7791: Add can_clk function

This patch adds can_clk function to r8a7743/r8a7791 which is cleaner,
and allows for independent configuration.
We keep the can_clk* pins definitions from within can0_groups and
can1_groups for uniformity and backwards compatibility.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>