ARM: dts: at91: sama5d2_xplained: Add QSPI0 + SPI NOR memory nodes

This patch enables the QSPI0 controller, configures its pin muxing and
declares a jedec,spi-nor memory.

sama5d2 Xplained RevB and RevC use the Macronix MX25L25673G flash
memory which advertises a maximum frequency of 80MHz for Quad IO
Fast Read. Set the spi-max-frequency to 80MHz knowing that actually
the QSPI drver will set the SPI bus clock to 166MHz / 3 = 55.3MHz.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@microchip.com>
Tested-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20200403061222.1277147-3-tudor.ambarus@microchip.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>