rockchip: clk: Add clk driver for rk3308

Add clk controller driver for RK3308 SOC.

This patch depends on Elaine's pll patch[0].

[0]http://patchwork.ozlabs.org/patch/1183718/



Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>