dt-binding: ARM: add clock binding docs for Marvell Berlin2 SoCs

This adds mandatory device tree binding documentation for the clock related
IP found on Marvell Berlin2 (BG2, BG2CD, and BG2Q) SoCs to the Berlin SoC
binding documentation.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>