mtd: maps: gpio-addr-flash: fix warnings and make more portable

As reported on lkml, building this module for HIMEM systems spews warnings
about mismatch in pointer types.  Further, we need to use ioremap() in order
to properly access the flash memory on most systems rather than just doing
it directly.

Reported-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>