clk: rockchip: make rk3308 ddrphy4x clock critical

Currently, no driver support for DDR memory controller (DMC) is present,
as a result, no driver is explicitly consuming the ddrphy clock. This means
that VPLL1 (parent of ddr clock) will be shutdown if we enable
and then disable any child clock of VPLL1 (e.g. SCLK_I2S0_8CH_TX).
If VPLL1 is disabled, the whole system will freeze, because the DDR
controller will lose its clock. So, it's necessary to prevent VPLL1 from
shutting down, by marking the ddrphy4x CLK_IS_CRITICAL.

This bug was discovered when I was porting rockchip_i2s_tdm driver to
mainline kernel from Rockchip 4.4 kernel. I guess that other Rockchip
SoCs without DMC driver may need the same patch. If this applies to
other devices, please let us know.

Signed-off-by: Yunhao Tian <t123yh@outlook.com>
Link: https://lore.kernel.org/r/BYAPR20MB24886765F888A9705CBEB70789E39@BYAPR20MB2488.namprd20.prod.outlook.com


[adapted subject, changed to add the clock to the critical list]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>