clk: sunxi: Add sun9i A80 cpus (cpu special) clock support

The "cpus" clock is the clock for the embedded processor in the A80.
It is also part of the PRCM clock tree. This clock includes a pre-
divider on one of its inputs. For now we are using a custom clock
driver for it. In the future we may want to develop a generalized
driver for these types of clocks, which also includes the AHB clock
driver on sun[5678]i.

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