dt-bindings: imx-gpcv2: Fix the unit address

In the provided example the unit address does not match the 'reg' value,
as IMX7_POWER_DOMAIN_PCIE_PHY is defined as 1.

Fix the unit address and avoid using defines in reg as per Rob
Herring's recommendation.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>