clk: at91: audio-pll: separate registration from DT parsing

Separate registration out of of_sama5d2_clk_audio_pll*_setup to allow other
code to use it.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
[sboyd@kernel.org: Include pmc.h]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>