tests: spi_loopback: exclude atmel sam0 boards

The spi loopback tests enables SPI ASYNC which for the SAM0 SPI driver
requires DMA support is configured.  None of the current SAM0 based
boards configure DMA for SPI and thus we exclude these boards as the
test will not compile for these boards.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>