MLK-14280: gpc: gpc driver not probed

MLK-14280: gpc: gpc driver not probed

The GPC controller driver is handling two sections: interrupts and PGC
(Power Gate Controller). The interrupts are handled in imx_gpcv2_init
function, and the PGC is handled in imx_gpcv2_probe function.
In kernel 4.9, the driver is probed by it's compatible entry in:
OF_DECLARE_2(irqchip, imx_gpcv2, "fsl,imx7d-gpc", imx_gpcv2_init);

Because the driver is already probed, imx_gpcv2_probe function is not
called, since it is registered with the same compatible name.
In order to separate the interrupts from PGC, this patch moves the
regulator entrys in DTS to a new node: pgc. And, assign the probe
function to this new node.
Also, added DeviceTree documentation about the new added node. Since GPC
(the block containing the PGC) didn't have any documentation, documented
GPC too.

Signed-off-by: Robert Chiras <robert.chiras@nxp.com>