ARM: dts: imx6sl: Convert gpc to new bindings

With old bindings imx_gpc_onecell_data always sets num_domains to 2 so
the DISPMIX domain can't actually be referenced. The pd is still defined
and pm core shuts it down as "unused" so display can't work.

Fix this by converting to new gpc bindings by adding pgc nodes and
referencing the newly-defined &pu_disp domain from &lcdif.

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