memory: atmel-ebi: switch to SPDX license identifiers

Adopt the SPDX license identifiers to ease license compliance
management.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20190906151519.19442-1-tudor.ambarus@microchip.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>