configs: sama5d2_xplained: add support QSPI flash boot

The spi-nor flash resides on spi bus 1. Update the CONFIG_ENV_SPI_CS
and CONFIG_BOOTCOMMAND accordingly.

Based on original work by Wenyou Yang.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
[tudor.ambarus@microchip.com: amend the commit message.]
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>