arm64: dts: renesas: beacon: Fix USB extal reference

The USB extal clock reference isn't associated to a crystal, it's
associated to a programmable clock, so remove the extal reference,
add the usb2_clksel.  Since usb_extal is referenced by the versaclock,
reference it here so the usb2_clksel can get the proper clock speed
of 50MHz.

Signed-off-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/20210513114617.30191-1-aford173@gmail.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>