ARM: dts: imx6sx: Add support for PCI power domain

Previously PCI support was working because the bootloader has previously
powered up the PCI power domain.

Represent the PCI power domain, so that PCI is functional without
relying on the PCI support from the bootloader.

Tested on a imx6sx-sdb board with no PCI support in the bootloader.

Reported-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>