soc: imx: gpc: Add i.MX6SX PCI power domain

i.MX6SX has a PCI power domain in PGC. Add support for it.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>