clk: rockchip: rk3036: fix implementation of usb480m clock mux

Contrary to how it is implemented right now, this mux is controllable via
a bit in CRU_MUSC_CON (same bit as on rk3128 even) and allows switching
between xin24m and the 480m output of the usb2phy.

So drop the hard-coded fixed-factor clock and implement the correct mux
instead.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250503202532.992033-3-heiko@sntech.de