m68k: io: Mark mmio read addresses as const

Passing constant __iomem tokens into the readl() family of helpers
or any of the others causes a warning on m68k:

    include/asm-generic/io.h: In function 'ioread8_rep':
    arch/m68k/include/asm/io_mm.h:375:44: warning: passing argument 1 of 'raw_insb' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
	 375 | #define readsb(port, buf, nr)     raw_insb((port), (u8

Add a 'const' modifier to the pointers to shut up the warnings here.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309251926.bPl23AhG-lkp@intel.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20230925155328.443664-1-arnd@kernel.org
[geert: Fix missed rom_in_{8,be16,le16}()]
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>