Documentation: bindings: update the Berlin controllers documentation

We're moving from a single node for multiple devices to a node with one
sub-node per sub-device, registered by simple-mfd. Update the
documentation to reflect the changes.

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