clk: renesas: rcar-gen3: Add support for mode pin clock selection

Make the existing support for selecting between clean and SSCG clocks
using MD12 more generic, to allow using other mode pins for arbitrary
clock selection.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>