mtd: rawnand: atmel: Select GENERIC_ALLOCATOR

The driver uses genalloc functions. Select GENERIC_ALLOCATOR to prevent
build errors when selected through COMPILE_TEST.

Fixes: 88a40e7d ("mtd: rawnand: atmel: Allow selection of this driver when COMPILE_TEST=y")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>