ARM: dts: berlin: convert BG2 to DT clock nodes

This converts Berlin BG2 SoC dtsi to make use of the new DT clock
nodes for Berlin SoCs. While at it, also fix up twdclk which is
running at cpuclk/3 instead of sysclk.

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