ARM: dts: meson8b: add the internal clock measurer

The Amlogic Meson8b SoC has an internal clock measurer IP which allows
measuring frequencies of various clock paths.
Enable it on meson8b.dtsi so we can use it.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>