Timer stop & start functions fix

[wiklam]

We want to set up the interrupts while attaching a timer and eventually stop it from invoking interrupts.
Removing unnecessary function tm_select (choosing time source is done while setting up timers in clock.c).
Typo in rtc.c.

[cahirwpz]

IIRC The intent of this PR is to change timer drivers so that resource allocation (including interrupt handler registration) is performed only when the driver is attached. This involves masking and unmasking an interrupt in *_start and *_stop timer routines. There's no infrastructure that allows for a single intr_handler_t to be temporarily disabled without tearing it down. Hence we resort to configuring timers in such a way they're effectively disabled. Is that correct?