boards: nrf: Indicate spi as supported on Nordic DK boards

tests/drivers/spi/spi_loopback provides configurations for boards
nrf51_pca10028 and nrf52840_pca10056, thus indicate spi as supported
on them, so that they are included in sanitycheck builds of this test.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>