drivers: timer: Move Timer API to unified interface

The CMSDK Timer can be used as a timer or as a counter.
The unified interface proposed in #8340 unifies counter.h and rtc.h to
provide a common interface.

This patch modifies the timer implementation of the single timer to
make it compliant with the new proposed interface.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>