drivers/mtd/nvmxip: Print phys_addr_t without warnings on both 32bit and 64bit systems

Cast the address such that it can be printed without warnings
on both 32bit and 64bit systems. This really should use some
better print formatter, but for the lack of it, do it this way.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>