ramips: fix usbphy DT nodes on linux 4.14

ramips: fix usbphy DT nodes on linux 4.14

The Ralink USB PHY driver merged into mainline has a slightly different
device tree binding than the patch that was used with linux 4.9.
The new driver requires a `ralink,sysctl` node pointing to the `syscon`
node.

This patch also sets `#phy-cells` to 0, as recommended by the mainline
documentation [1].

[1] Documentation/devicetree/bindings/phy/ralink-usb-phy.txt

Signed-off-by: Vianney le Clément de Saint-Marcq <[email protected]>