ARM: dts: imx6sl: Add power-domain information to gpc node

The PGC that is part of GPC controls isolation and power sequencing of the
power domains. The PU power domain will be handled by the generic pm domain
framework. It needs a phandle to the PU regulator to turn off power when
the domain is disabled and a list of clocks to be enabled during powerup
for reset propagation.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>