Since commit 879c0e5e ("ARM: imx: Remove i.MX27 board files")
the following W=1 build warning is seen:

arch/arm/mach-imx/pm-imx27.c:40:13: warning: no previous prototype for 'imx27_pm_init' [-Wmissing-prototypes]

Fix it by including the "common.h" header file, which
contains the prototype for imx27_pm_init().

Fixes: 879c0e5e ("ARM: imx: Remove i.MX27 board files")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>