MLK-11249-2 ARM: imx: add support for dynamic settings of gpc timing

On different platforms, the CPU power down/up timing may be
different because of different requirements or different
implementations in hardware, so we need to support dynamic
setting of these timing, board level dtb file will provide
these settings, and GPC driver need to read them and set them
to the hardware.

Signed-off-by: Anson Huang <b20788@freescale.com>