dt-bindings: clock: rk3036: add SCLK_USB480M clock-id

Contrary to how it is implemented right now, the usb480m clock is a
controllable mux that can switch between the 24MHz oscillator and the
clock output of the usb2phy.

Add the needed clock-id to allow setting this mux from DT.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250503202532.992033-2-heiko@sntech.de