spi: Add Atmel QuadSPI driver

Backport the driver from linux v5.1-rc5 and adapt it for u-boot.
Tested on sama5d2_xplained Rev B with mx25l25635e spi-nor flash.

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