clk: sunxi: Add PLL3 clock

The A10 SoCs and relatives have a PLL controller to drive the PLL3 and
PLL7, clocked from a 3MHz oscillator, that drives the display related
clocks (GPU, display engine, TCON, etc.)

Add a driver for it.

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