ARM: dts: sun7i: Add cpu clock reference and operating points to dtsi

The cpu core is clocked from the "cpu" clock. Add a reference to it
in the first cpu node. Also add "cpu0" label to the node.

The operating points were taken from the A20 FEX files in the
sunxi-boards repository. Not all boards have the same settings. The
settings in this patch are the most generic ones.

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