mtd: rawnand: atmel: Add an __iomem cast on gen_pool_dma_alloc() call

gen_pool_dma_alloc() return type is void *, while internally, the
memory region exposed by the sram driver has been mapped with
ioremap().

Add a void * to void __iomem * cast to make sparse happy.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>