clk: sunxi-ng: Add A31/A31s clocks

Add a new style driver for the clock control unit in Allwinner A31/A31s.

A few clocks are still missing:

    - MIPI PLL's HDMI mode support
    - EMAC clock

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>