Fix Espressif timer registering ID. Espressif's general purpose
timers are grouped in different Timer Groups, which may contain
more than one general purpose timer. The current implementation is
related to the timer groups (timer0 refers to Timer Group 0, for
instance), but previous registering method was referring to the
timer inside each group, which caused it to fail when more than
one timer group was being selected because only the first timer
on each group is being actually registered.
Signed-off-by: Tiago Medicci Serrano <tiago.medicci@espressif.com>