iio: max31856: add option for setting mains filter rejection frequency

This sensor has an embedded notch filter for reducing interferences caused
by the power mains. This filter can be tuned to reject either 50Hz or 60Hz
(and harmonics).

Currently the said setting is left alone (the sensor defaults to 60Hz).
This patch introduces a IIO attribute that allows the user to set the said
filter to the desired frequency.

NOTE: this has been intentionally not tied to any DT property to allow
the configuration of this setting from userspace, e.g. with a GUI or by
reading a configuration file, or maybe reading a GPIO tied to a physical
switch or accessing some device that can autodetect the line frequency.

Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Patrick Havelange <patrick.havelange@essensium.com>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Matt Ranostay <matt.ranostay@konsulko.com>
Cc: Chuhong Yuan <hslester96@gmail.com>
Cc: Daniel Gomez <dagmcr@gmail.com>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>