ARM: dts: at91: add unit-address to memory node

The memory node requires a unit-address, add it.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20200831171129.3886857-7-alexandre.belloni@bootlin.com