ARC: [dts] Introduce Timer bindings

ARC Timers have historically been probed directly.
As precursor to start probing Timers thru DT introduce these bindings
Note that to keep series bisectable, these bindings are not yet used in
code.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>