ARM: mmp: add platform_device head file in gplugd

arch/arm/mach-mmp/gplugd.c: In function ‘gplugd_init’:
arch/arm/mach-mmp/gplugd.c:188:2: error: implicit declaration of
function ‘platform_device_register’
[-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors
make[1]: *** [arch/arm/mach-mmp/gplugd.o] Error 1
make: *** [arch/arm/mach-mmp] Error 2

So append platform_device.h to resolve build issue.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>