External clock input for Atmel SAMD21
Answered my own question in the datasheet! Looks like setting the XTALEN bit in the XOSC32K register of the SYSCTRL section to 0 solves the question:

Also from the datasheet:

The external clock or crystal oscillator is enabled by writing a one
to the Enable bit (XOSC32K.ENABLE) in the 32kHz External Crystal
Oscillator Control register. To enable the XOSC32K as a crystal
oscillator, a one must be written to the XTAL Enable bit
(XOSC32K.XTALEN). If XOSC32K.XTALEN is zero, external clock input will
be enabled.