iio: core: move 'mlock' to 'struct iio_dev_opaque'

Now that there are no more users accessing 'mlock' directly, we can move
it to the iio_dev private structure. Hence, it's now explicit that new
driver's should not directly use this lock.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20221012151620.1725215-5-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

iio: add a TODO

This patch adds a TODO file with some work items added with reference to
the conversation in [1].
A TODO file is immensely useful while onboarding new contributors who
are looking for some low hanging fruit to get their foot into the door.
Since these items affect all drivers the file has been placed in the
root iio directory instead of augmenting the staging TODO.

Thanks,
Rohit

[1]: https://marc.info/?l=linux-iio&m=158256721009892&w=2
Signed-off-by: Rohit Sarkar <rohitsarkar5398@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>