imx: power-domain: Descend into pgc subnode if present

imx: power-domain: Descend into pgc subnode if present

In case the power domain node structure is gpc@303a0000/pgc/power-domain@N,
do not bind power domain driver to the 'pgc' node, but rather descend into
it and only bind power domain drivers to power-domain@N subnodes. This way
we do not waste one useless driver instance associated with 'pgc' node.

Tested-By: Tim Harvey <tharvey@gateworks.com> #imx8mp-venice-defconfig
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>