dt-bindings: clock: meson8b: export the CPU post dividers

There are four CPU clock post dividers:
- ABP
- PERIPH (used as input for the ARM global timer and ARM TWD timer)
- AXI
- L2 DRAM

Export these so we can use them in .dts files.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20181122214017.25643-2-martin.blumenstingl@googlemail.com