Documentation: bindings: move the Berlin clock documentation

The Berlin clock documentation was part of the Marvell Berlin SoC
documentation because the Berlin clock configuration was inside the
chip controller. With the recent rework of the chip and system
controller handling (now all sub-devices of the soc and system
controller nodes are registred with simple-mfd, and each device has its
own sub-node), the documentation of the Berlin clock driver can be moved
to the generic clock documentation directory.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>