clk: berlin: add core clock driver for BG2Q

This driver deals with the core clocks found on Marvell Berlin BG2Q. 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: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>