clk: rockchip: add ability to specify pll-specific flags

This adds a flag parameter to plls that allows us to create
special flags to tweak the behaviour of the plls if necessary.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Kever Yang <kever.yang@rock-chips.com>