clk: berlin: add driver for BG2x complex divider cells

This is a driver for the complex divider cells found on Marvell Berlin2
SoCs. The cells come in two flavors: single register cells and shared
register cells.

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>