ARM: sun7i: Add audio PLL

The A20 uses the PLL2 as the audio PLL, which is the parent of all the
other audio clocks in the system (i2s, codec, etc.). Add it to the DTSI.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>