Documentation: bindings: move the Berlin pinctrl documentation

The Berlin pinctrl documentation was part of the Marvell Berlin SoC
documentation because the Berlin pinctrl configuration was inside the
chip and the system controllers. With the recent rework of the chip and
system controller handling (now an MFD driver registers all sub-devices
of the two soc and system controller nodes and each device has its own
sub-node), the documentation of the Berlin pinctrl driver can be moved
to the generic pinctrl documentation directory.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>