clk: rockchip: Allow sclk_i2s0 and i2s0_frac to change their parents rate on rk3188

Allow sclk_i2s0 and i2s0_frac to change their parents rate as
that the upstream dividers are purely there to feed sclk_i2s0

Tested on radxarock-lite.

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>