ARM: dts: sun7i: external clock outputs

This commit adds the two external clock outputs available on A20 to
its device tree. A dummy fixed factor clock is also added to serve as
the first input of the clock outputs, which according to AW's A20 user
manual, is the 24MHz oscillator divided by 750.

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