ARM: mvebu: db-88f6820-amc: enable SPI_FLASH_BAR

This board uses Micron N25Q256A SPI flash. Enable SPI_FLASH_BAR to allow
us to access the whole chip.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Cc: Vignesh R <vigneshr@ti.com>
Signed-off-by: Stefan Roese <sr@denx.de>