arm64: defconfig: Enable usb2_clksel for R-Car Gen3 and RZ/G2

There is a clock driver that's needed when using an external
USB reference clock called usb2_clksel.
Enable it in the defconfig.

Signed-off-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/20210513114617.30191-3-aford173@gmail.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>