clk: berlin: add cpuclk

Add cpuclk in the Berlin BG2Q clock driver. This clk has a divider
fixed to 1.

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>