clk: renesas: rcar-gen3: Add support for OSC EXTAL predivider

Add a clock type and macro for defining clocks using the OSC EXTAL
predivider combined with a fixed divider.

On most R-Car Gen3 SoCs, the predivider value depends on mode pins, and
thus must be specified in the configuration structure.

Inspired by a patch in the BSP by Takeshi Kihara
<takeshi.kihara.df@renesas.com>.

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