clk: renesas: rcar-gen3: Add support for RCKSEL clock selection

Add a clock type and macro for defining clocks where the parent and
divider are selected based on the value of the RCKCR.CKSEL bit.

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