soc: atmel_sam0: Add GCLK Reset
soc: atmel_sam0: Add GCLK Reset

Fixes an issue with mcuboot where the SoC will freeze when the dpll is
disabled since `atmel_samd_init` was already run in the bootloader.

Signed-off-by: Abram Early <a14ea99d94c4abb2faed72e549f125cb2a39005f@gmail.com>

finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr