rockchip: px30: add support for setting cpll clock

Starting with commit 92f1e9a4 ("clk: Detect failure to set
defaults") the clk driver for the PX30 for the Odroid Go Advance would
no longer probe correctly, because setting the cpll and gpu clocks are
not supported with the clk_px30 U-Boot driver. This adds support for
setting the cpll clock to the clk_px30 driver. Another patch will
update the U-Boot specific device-tree to remove the GPU clock which is
not used by U-Boot.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>