drivers: timer: update TIMER_IRQ for tests/kernel/context

This test has gotten out of control.  It has a giant #if cascade
enumerating every timer driver in the Zephyr tree and extracting its
interrupt number.  Which means that every driver needs to somehow
expose that interrupt in its platform headers or some other API.

Make it a simple integer variable exported by the timer driver for the
benefit of this one test.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>