arm64: Include linux/io.h in mm/mmap.c

arm64: Include linux/io.h in mm/mmap.c

Commit 7165d09cd24b ("arm64: mm: Remove unused header file") removed
a bunch of apparently "unused" header inclusions from our mm/mmap.c
implementation, but in doing so introduced the following warning when
building with W=1:

>> arch/arm64/mm/mmap.c:17:5: warning: no previous prototype for 'valid_phys_addr_range' [-Wmissing-prototypes]
      17 | int valid_phys_addr_range(phys_addr_t addr, size_t size)
         |     ^~~~~~~~~~~~~~~~~~~~~
>> arch/arm64/mm/mmap.c:36:5: warning: no previous prototype for 'valid_mmap_phys_addr_range' [-Wmissing-prototypes]
      36 | int valid_mmap_phys_addr_range(unsigned long pfn, size_t size)
         |     ^~~~~~~~~~~~~~~~~~~~~~~~~~

Add back the linux/io.h header inclusion to pull in the missing
prototypes.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/202101271438.V9TmBC31-lkp@intel.com
Signed-off-by: Will Deacon <will@kernel.org>