clk: sunxi-ng: Wait for lock when using fractional mode

Currently ccu_frac_helper_set_rate() doesn't wait for a lock bit to be
set before returning. Because of that, unstable clock may be used.

Add a wait for lock in the helper function.

Fixes: 89a3dfb7 ("clk: sunxi-ng: Add fractional lib")
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>