clk: rockchip: fix the incorrect pclk_edp div width for RK3399

The range of the  pclk_edp_div_con is [13:8] and 6 bits, not 5.

Reported-by: Lin Huang <hl@rock-chips.com>
Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>
Tested-by: Lin Huang <hl@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>