clk: renesas: r8a77980: Add OSC predivider configuration and clock

R-Car Gen3 Hardware Manual Rev.0.54 documents the relation between the
MD13 and MD14 mode pins, and the OSC EXTAL predivider, as used by the
OSC clock.  Hence augment the configuration structure with all
documented predivider values.

Add the OSC clock using the configured predivider.

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