hw/timer/altera_timer.c: Switch to transaction-based ptimer API

hw/timer/altera_timer.c: Switch to transaction-based ptimer API

Switch the altera_timer code away from bottom-half based ptimers to
the new transaction-based ptimer API.  This just requires adding
begin/commit calls around the various places that modify the ptimer
state, and using the new ptimer_init() function to create the timer.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: 20191017132905[email protected]