[MIPS] Workaround for a sparse warning in include/asm-mips/io.h

CKSEG1ADDR() returns unsigned int value on 32bit kernel.  Cast it to
unsigned long to get rid of this warning:

include2/asm/io.h:215:12: warning: cast adds address space to expression (<asn:2>)

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>