mtd: rawnand: fsmc: Use uintptr_t casts instead of unsigned ones

uintptr_t should be used when casting a pointer to an unsigned int so
that the code compiles without warnings even on 64-bit architectures.

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

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