phy: phy-rockchip-inno-usb2: support standalone phy nodes

New Rockchip devices have the usb2 phy devices as standalone nodes
instead of children of the grf node.
Allow the driver to find the grf node from a phandle.

Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Tested-by: Michael Riesch <michael.riesch@wolfvision.net>
Link: https://lore.kernel.org/r/20211215210252.120923-5-pgwipeout@gmail.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>