Timer does not build if NUM_TIMERS is set to 1

The timer will fail to build with a vague error of the form
```
ERROR: [Synth 8-523] part-select [3:4] does not match declaration apb_timer/src/apb_timer.sv:37]
```
... if NUM_TIMERS is set to 1. This seems to be an edge case that is unhandled due how `$clog2` is used.