ARM: imx: mmdc: Include "common.h" header file

Include the "common.h" header file to fix the following sparse warning:

arch/arm/mach-imx/mmdc.c:66:5: warning: symbol 'imx_mmdc_get_ddr_type' 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>