ARM: imx: add L2 page power control for GPC

Some platforms like i.MX6UL/i.MX6SLL have L2
page power control in GPC, it needs to be
disabled if ARM is power gated and L2 is NOT
flushed, add GPC interface to control it.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>