dt-bindings: clock: meson8b: add APB clock definition

dt-bindings: clock: meson8b: add APB clock definition

Commit 8e1dd17c8b0e3f ("dt-bindings: clock: meson8b: export the CPU
post dividers") added a clock with the name "ABP". The actual name of
this clock is "APB".

Add a new #define with the same ID but the correct name. The old #define
will be dropped in a follow-up patch because each commit in the tree
must compile on it's own (the old #define is still used by the clock
controller driver).

Fixes: 8e1dd17c8b0e3f ("dt-bindings: clock: meson8b: export the CPU post dividers")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lkml.kernel.org/r/20190210222603.6404-2-martin.blumenstingl@googlemail.com