clk: sunxi: add clock-output-names dt property support

sunxi clock drivers use dt node name as clock name, but clock
nodes should be named clk@X, so the names would be the same.
Let the drivers read clock names from dt clock-output-names
property.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Emilio López <emilio@elopez.com.ar>