clk: sunxi: support better factor DT nodes

The DT nodes should look like

    abc_clk: clk@deadbeef {
        ...
        clock-output-names = "abc";
    }

But our old DT nodes look like

    abc: abc@deadbeef {
        ...
    }

So, let's support both formats, until we can transition everything
to the new, correct one.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Acked-by: Mike Turquette <mturquette@linaro.org>