clk: rockchip: protect the pclk_usb_grf as critical on px30

Make this clock a real critical clock, so that writes to the usbphy grf
always succeed.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Link: https://lore.kernel.org/r/20190917081903.25139-5-heiko@sntech.de