arm64: dts: imx8mn: add GPC node

Add the DT node for the GPC, including all the PGC power domains,
some of them are not fully functional yet, as they require interaction
with the blk-ctrls to properly power up/down the peripherals.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>