clk: sunxi: update clock-output-names dt binding documentation

clock-output-names is now required for most of sunxi clock nodes, to
provide the name of the corresponding clock. Add the new requirements,
exceptions, as well as examples.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Emilio López <emilio@elopez.com.ar>