dt-bindings: timer: Document Owl timer

The Actions Semi S500 SoC contains a timer block with two 2 Hz and two
32-bit timers. The S900 SoC timer block has four 32-bit timers.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>