clk: rockchip: rk3036: mark ddrphy as critical

The ddrphy is supplied by the dpll, but due to the limited number of PLLs
on the rk3036, the dpll also is used for other periperhals, like the GPU.

So it happened, when the Lima driver turned off the gpu clock, this in
turn also disabled the dpll and thus the ram.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250503202532.992033-4-heiko@sntech.de