staging:iio:hmc5843: Add trigger handling

v3:
* use __be16 instead of s16
v2 (thanks to Jonathan Cameron):
* drop dynamic buffer allocation, buffer is in hmc5842_data
* grab timestamp near data acquisition
* restrict available scan masks (only read all axis)

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>