ARM: dts: imx7s: add mipi phy power domain

Add power domain index 0 related with mipi-phy to imx7s.

While at it rename pcie power-domain node to remove pgc prefix.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>