ARM: dts: imx6sx: Add DISPLAY power domain support

This was implemented in the driver but not actually defined and
referenced in dts. This makes it always on.

From reference manual in section "10.4.1.4.1 Power Distribution":

"Display domain - The DISPLAY domain contains GIS, CSI, PXP, LCDIF,
PCIe, DCIC, and LDB. It is supplied by internal regulator."

The current pd_pcie is actually only for PCIE_PHY, the PCIE ip block is
actually inside the DISPLAY domain. Handle this by adding the pcie node
in both power domains.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>