Adds two board config define's, which can be added in a board's
mpconfigboard.h: BOARD_XOSC_FREQ_HZ, and BOARD_XOSC_IS_CRYSTAL, which
are passed to clock_init(). External clock sources are currently only
implemented for SAM_D5X_E5X series chips, so defining BOARD_XOSC_FREQ_HZ
for a SAMD21 board will emit an error.
Signed-off-by: Qyriad <qyriad@qyriad.me>
3f3ec1c835