counter: nxp_pit: support only top callback

This periodic timer (PIT) has a single load value register for each
channel, which is currently used for both alarm and top callback APIs.
If using both APIs together (which is a valid use-case) it will write
to the same register causing unexpected behavior of the timer.

The nature of the PIT is to trigger an event (like interrupts) at a
certain rate, and not to produce single-shot events. Hence keep only top
callback functionality.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>