configs: sama5d2_xplained: add qspiflash_defconfig

Add the default config file of QSPI media. The config is based on
sama5d2_xplained_mmc_defconfig.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>