mtd: onenand: samsung: Fix pointer cast -Wpointer-to-int-cast warnings on 64 bit

iomem pointers should be casted to unsigned long to avoid
-Wpointer-to-int-cast warnings when compiling on 64-bit platform (e.g.
with COMPILE_TEST):

    drivers/mtd/nand/onenand/samsung_mtd.c: In function ‘s3c_onenand_readw’:
    drivers/mtd/nand/onenand/samsung_mtd.c:251:6: warning:
        cast from pointer to integer of different size [-Wpointer-to-int-cast]
      if ((unsigned int) addr < ONENAND_DATARAM && onenand->bootram_command) {
          ^

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>