dt-bindings: Add GPCv2 power gating driver

Add DT bindings for power domain driver for GPCv2 IP block found in
i.MX7 SoCs.

Cc: yurovsky@gmail.com
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Dong Aisheng <dongas86@gmail.com>
Cc: devicetree@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>