boards: atmel soc: Remove non-minimal peripherals from defconfig

Some boards, e.g. atsamd20_xpro, enable non-minimal peripherals by
default like SPI.  This goes against board porting guidelines, as it
enables a peripheral that is not necessarily used by all samples,
as discussed at #30694.  This removes SPI as a default peripheral
for all sam/sam0 boards.

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