clk: sunxi-ng: sun5i: Fix bit offset of audio PLL post-divider

The post-divider for the audio PLL is in bits [29:26], as specified
in the user manual, not [19:16] as currently programmed in the code.
The post-divider has a default register value of 2, i.e. a divider
of 3. This means the clock rate fed to the audio codec would be off.

This was discovered when porting sigma-delta modulation for the PLL
to sun5i, which needs the post-divider to be 1.

Fix the bit offset, so we do actually force the post-divider to a
certain value.

Fixes: 5e737617 ("clk: sunxi-ng: Add sun5i CCU driver")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>