boards/arm/serpente: fix system clock rate

Serpente schematic has the same clocking arrangement as
boards/arm/adafruit_trinket_m0.  This fix copies the clocking
config from the trinket.

Fixes https://github.com/zephyrproject-rtos/zephyr/issues/64470



Signed-off-by: Tom Keddie <github@bronwenandtom.com>