tests: drivers: spi: adp_xc7k_ae350.conf for SPI_LARGE_BUFFER_SIZE

Due to the hardware transmission limitation of SPI in adp_xc7k_ae350
(maximum of 512 bytes per transfer), CONFIG_SPI_LARGE_BUFFER_SIZE is
specified in the board configuration in the spi_loopback test.

Signed-off-by: Kevin Wang <kevinwang821020@google.com>