ARM: dts: meson8b: add the Cortex-A5 global timer

The Meson8b SoC is using four Cortex-A5 cores. These come with an ARM
global timer.
This adds the Cortex-A5 global timer but keeps it disabled for now. The
timer is clocked by the "PERIPH" clock whose rate can change during
runtime (when changing the frequency of the CPU clock). Unfortunately
the arm_global_timer driver does not handle changes to the clock rate
yet.

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