ARM: imx: fix missing include of common.h

Fix the following warning by including ../common.h to provide
the protoype for mxc_register_gpio() :

arch/arm/mach-imx/devices/platform-gpio-mxc.c:11:24: warning: symbol 'mxc_register_gpio' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>