ARM: imx: add cpu idle support for i.MX6SLL

i.MX6SLL supports cpu idle with ARM power gated,
it can reuse i.MX6SX's cpu idle driver to support
below 3 states of cpu idle:

state0: WFI;
state1: WAIT mode with ARM power on;
state2: WAIT mode with ARM power off.

L2_PGE in GPC_CNTR needs to be cleared to support
state2 cpu idle.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>