tty: serial: meson: Make some bit of the REG5 register writable

Make the internal clock source mux and divider writeable, allowing the
uart to deviate from the settings intially applied by the ROMCode and
using the most appropriate clocks.

Signed-off-by: Yu Tu <yu.tu@amlogic.com>
Link: https://lore.kernel.org/r/20220225073922.3947-5-yu.tu@amlogic.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>