clk: rockchip: Limit use of USB PHY clock to USB on rk3288

The USB PHY clock can be configured as (grand) parent of uart0_sclk and
sclk_gpu. It has been observed that UART0 doesn't work reliably in high
speed mode with the PHY clock as input when certain USB devices are
plugged to the USB HOST1 port (see https://crrev.com/c/320543

).

Prefix the name of the PHY clock with a '.' in the non-USB muxes to
effectively remove the clock as input from these muxes.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>