MLK-12262-4 ARM: imx: fix low power idle issue on i.MX7D TO1.1

MLK-12262-4 ARM: imx: fix low power idle issue on i.MX7D TO1.1

For low power idle with ARM power gated, per hardware requirement,
there must be no interrupt coming during the power down
process of ARM core, so RBC counter is enabled to hold interrupts
and GIC must be disabled at the moment;

The hardware design team recommends ~240us is required during ARM
core power down, so we update the RBC counter value to 8(~240us);

Update GPC SCU and CPU power up/down timing according to design
team's recommendation.

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