tests: spi_loopback: Add atsame54_xpro configuration

Adds a board-specific configuration for the atsame54_xpro to the
spi_loopback test.

Signed-off-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>
diff --git a/tests/drivers/spi/spi_loopback/boards/atsame54_xpro.conf b/tests/drivers/spi/spi_loopback/boards/atsame54_xpro.conf
new file mode 100644
index 0000000..8497667
--- /dev/null
+++ b/tests/drivers/spi/spi_loopback/boards/atsame54_xpro.conf
@@ -0,0 +1,2 @@
+CONFIG_SPI_LOOPBACK_DRV_NAME="SPI_0"
+CONFIG_SPI_LOOPBACK_CS_GPIO=n