clk: rockchip: add support for half divider

The new Rockchip socs have optional half divider:
The formula is shown as:
	freq_out = 2*freq_in / (2*div + 3)
Is this the same for all of new SoCs.

So we use "branch_half_divider" + "COMPOSITE_NOMUX_HALFDIV \
DIV_HALF \ COMPOSITE_HALFDIV \ CMPOSITE_NOGATE_HALFDIV"
to hook that special divider clock-type into our clock-tree.

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