clk: berlin: add core clock driver for BG2/BG2CD

This driver deals with the core clocks found on Marvell Berlin
BG2 and BG2CD. For the shared register dividers, make use of the
corresponding driver and add some single clock muxes and gates for
the rest.

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