dts/bindings: Remove #{addr,size}-cell props from st,stm32-timers

The binding for st,stm32-timers specifies #address-cells and #size-cells
as required but no dts files that have st,stm32-timers specify these
properties.  Remove them from the binding.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>