boards: arm: sam0: Fix pinmux init prio

After pinmux being converted to build a separate static library
console not show anymore and there is a hard fault.  The hard
fault happen because pinmux drivers are not ready yet and board
emits an assert.  This change board pinmux initialization to
PRE_KERNEL_2 and let system take care error handling.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>