ARM: mxs: mm: Fix sparse warning

Include <mach/common.h> header to fix the following sparse warnings:

arch/arm/mach-mxs/mm.c:43:13: warning: symbol 'mx23_map_io' was not declared. Should it be static?
arch/arm/mach-mxs/mm.c:48:13: warning: symbol 'mx28_map_io' was not declared. Should it be static?

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>