This uses the crystal to clock the RTC on boards which have a crystal.
Disable clock generator 2 which was enabled in commit
8e2080411f ("atmel-samd: Add rtc module support").
samd51 differs from samd21 when it comes to the RTC clock. samd51 doesn't
have an explicit clock peripheral so no need for a clock generator.
The same commit didn't even setup XOSC32K correctly, it missed EN1K and XTALEN.
The RTC uses the 1k clock output, so enable it on the OSCULP32K even if it works without it.
