ARM: berlin: dts: add CLKID_CPU for BG2Q

Marvell Berlin BG2Q SoC also has a clock for the CPU, add a
corresponding CLKID to the dt-binding include.

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