Per the reference manual for the RZ/G Series, 2nd Generation,
the RZ/G2M, RZ/G2N, and RZ/G2H have a bit that can be set to
choose between a crystal oscillator and an external oscillator.

Because only boards that need this should enable it, it's marked
as disabled by default for backwards compatibility with existing
boards.
Signed-off-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/20201228202221.2327468-2-aford173@gmail.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>