dt-bindings: soc: rockchip: drop unnecessary #phy-cells from grf.yaml

The recent yaml conversion of the grf and inno-usb2-phy bindings
left the #phy-cells in place in the main usb2phy node inside the
example in grf.yaml, causing new warnings.

Drop it to make the bindingcheck happy.

Fixes: e71ccdff ("dt-bindings: phy: rename phy nodename in phy-rockchip-inno-usb2.yaml")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20210607141845.3331910-1-heiko@sntech.de