mtd: rawnand: atmel: Use uintptr_t casts instead of unsigned int

When casting a pointer to an unsigned int, uintptr_t should be used to
cope with the pointer size differences between 32-bit and 64-bit
architectures.

This is needed if we want to allow compilation of this driver when
COMPILE_TEST=y.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>