soc: imx: move PGC handling to a new GPC driver

This is an almost complete re-write of the previous GPC power gating control
code found in the IMX architecture code. It supports both the old and the new
DT binding, allowing more domains to be added later and generally makes the
driver easier to extend, while keeping compatibility with existing DTBs.

As the result, all functionality regarding the power gating controller
gets removed from the IMX architecture GPC driver.  It keeps only the
IRQ controller code in the architecture, as this is closely coupled to
the CPU idle implementation.

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