drivers: counter native: Add top value conf and multi channel support

The counter_native_posix driver currently does not support top value
configuration, i.e. `ctr_set_top_value` returns `-ENOTSUP`. This commit
adds support for top value configuration, and with the counter API now
fully implemented, adds `counter` to `supported` peripherals for
native_posix target.
It also resolves an existing bug in which the
counter ISR did not reset upon reaching `TOP_VALUE`.
And adds support for multiple channels

Signed-off-by: Jason Wright <jason@jpw.nyc>
Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>