Some platforms are missing timer channels, this is now left to the
(missing) preprocessor definitions on those platforms, no
platform-specific defines needed anymore.
Also fix usage of timer counter register 3 (hardcoded) in
cpu/avr/dev/clock.c -- this code isn't used on many platforms as it
requires a very special quartz clock frequency but this now also uses
the platform timer specification.
b6e20bf6c0