mtd: rawnand: davinci: Stop doing iomem pointer <-> u32 conversions

There is no point in doing this sort of conversion since we can
replace |= by += operations which are perfectly valid on pointers.

This is done in preparation of COMPILE_TEST addition to the NAND_DAVINCI
Kconfig entry, since building for x86 generates several warnings because
of inappropriate u32 <-> void * conversions (pointers are 64-bit large
on x86_64).

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>