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 zephyrproject-rtos#64470

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

boards/arm/serpente/serpente_defconfig

@@ -3,6 +3,8 @@
 CONFIG_SOC_SERIES_SAMD21=y
 CONFIG_SOC_PART_NUMBER_SAMD21E18A=y
 CONFIG_BOARD_SERPENTE=y
+CONFIG_SOC_ATMEL_SAMD_OSC8M=y
+CONFIG_SOC_ATMEL_SAMD_OSC8M_AS_MAIN=y
 CONFIG_CONSOLE=y
 CONFIG_GPIO=y
 CONFIG_UART_CONSOLE=y