ARM: imx6: gpc: don't register power domain if DT data is missing

If the devicetree is too old and does not provide the regulator and clocks
for the power domain, we need to avoid registering the power domain.
Otherwise runtime PM will try to control the domain, which will lead to
machine hangs without the proper DT configuration data.

This restores functionality to the kernel 4.0 level if an old DT is
detected, where the power domain is constantly powered on.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>