clk: rockchip: mark pclk_ddrupctl as critical_clock on rk3036

No driver to handle this clk yet, but chip design requiress for this clock
supplying the ddr controller to be always on.

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>