ARM: dts: meson8: add the internal clock measurer

The Amlogic Meson8 SoC has an internal clock measurer IP which allows
measuring frequencies of various clock paths.
Enable it on meson8.dtsi so we can use it.

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