timer: Tidy up use of notrace

Tracing is typically enabled by the time driver model starts up, so there
is no point in adding a 'notrace' to the timer-init function. However,
once the driver model timer is enabled, we do need to be able to access
the timer's private data when reading the timer, so add it to the core
function needed for that.

Update the function's documentation while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>