ARM: dts: am335x-chiliboard: add /chosen/tick-timer

Commit 4b2be78a ("time: Fix get_ticks being non-monotonic") has
broken boot on chiliboard platform, as it requires '/chosen/tick-timer'
in device-tree. This resulted in following panic message:

  Could not initialize timer (err -19)

Provide missing chosen property in device-tree to fix chiliboard
support.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>