We occasionally have some public `foo.h` header that includes a private
`foo_arch.h` header. Users are expected to include the `foo.h` header
and not the `foo_arch.h`. However, clangd will claim that the `#include`
of `foo.h` is unused if only functions / macros/ types / ... from
`foor_arch.h` is used and nothing from `foo.h`.
This adds the `IWYU pragma: export` comment to the include of
`foo_arch.h` in `foo.h`, so that clangd treats functions / macros /
types provided by `foo_arch.h` as if they were instead provided by
`foo.h`, which fixes the false positives.
Allow accessing supported timer frequencies with a dedicated API.
This API needs to be implemented per platform and is available with
the feature periph_timer_query_freqs.
The default driver type is just an index into a device array defined
by the board.
If a platform wants to encode additional information in the device type,
it can define a custom type.
This means we can just set the default type to whatever fits the target
CPU best.
On ARM this will still be a 32 bit word, but on AVR it will by a 8 bit byte.
Most timers are implemented this way already, and keeping (documenting)
it that way allows the generic timer_set implementation to stay as
simple as it is.
Change documentation on return codes in periph/timer API to
return 0 on success and (-1) on error by default.
For timer_init this was already the case, but for timer_set,
timer_set_absolute, and timer_clear this is now changed
from 1 to 0 for success, while error remains (-1).
