ARM: dts: sun6i: 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 a list compiled by Maxime Ripard,
which is based on A31 FEX files from the sunxi-boards repository. Not all
boards have the same settings. The settings in this patch are the ones
shared by A/B/C revisions, plus the default clock setting from u-boot.

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