ARM: dts: meson8: add the ARM TWD timer

The Meson8 and Meson8m2 SoC are using four ARM Cortex-A9 cores which
come with a "TWD" (Timer-Watchdog) based timer. This adds support for
the ARM TWD Timer on these two SoCs.

Suggested-by: Carlo Caione <carlo@endlessm.com>
[ rebased patch from Carlo, use IRQ_TYPE_EDGE_RISING instead of
  IRQ_TYPE_LEVEL_LOW to prevent "GIC: PPI13 is secure or misconfigured"
  message during boot, use pre-processor macros to specify the IRQ,
  added the correct clock, dropped TWD watchdog node since there's no
  driver for it anymore ]
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>