clk: berlin: add driver for BG2x audio/video PLL

This is a driver for the AVPLLs built upon a VCO with 8 channels each
found on Marvell Berlin2 SoCs. While both VCOs found on BG2/BG2CD share
the same register set, sometimes registers shifts for one of the VCOs
are a bit off. Nothing serious that should require a separate driver,
so deal with both VCOs in a single driver instead.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>