Implement custom timer

[geonnave]

We want a timer API that allows setting time references, cancelling or shifting the timer, and connecting to a PPI channel.

Initial API:

    * set_oneshot_us(timer_ch, time_ref, us, cb) -> used for tte2, rt2, rte3
    * set_oneshot_us_ppi(timer_ch, time_ref, us, ppi_ch, task) -> used for tt1 and rt1
    * fix_drift(timer_ch, drift)
    * cancel(timer_ch)

[geonnave]

All except PPI is solved. Close and address PPI in separate issue #32.