pinctrl: renesas: Implement get_pin_muxing() callback

Implement get_pin_muxing() callback so that pinmux status
command can be used on Renesas platforms.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>