soc: imx: gpc: fix power up sequencing

Currently we wait only until the PGC inverts the isolation setting
before disabling the peripheral clocks. This doesn't ensure that the
reset is properly propagated through the peripheral devices in the
power domain.

Wait until the PGC signals that the power up request is done and
wait a bit for resets to propagate before disabling the clocks.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>