staging: iio: add support for hmc5883/hmc5883l to hmc5843 magnetometer driver

v3 addresses review comments:
* rotate tables (Jonathan Cameron)
* remove trailing space, add newline in sysfs output (Jonathan Cameron)
* split out patch for reorganization of hmc5843_set_rate() (Jonathan Cameron)
* use static table to describe chip variants (Jonathan Cameron)

v2 addresses review comments:
* fixes and cleanups have been split out (Jonathan Cameron)
* constants are generally prefixed HMC5843_, except when related
  specifically to hmc5883 (Jonathan Cameron)
* simplify code and avoid temp buffer in
  hmc5843_show_sampling_frequencies_available() (Lars-Peter Clausen)
* use sysfs_streq() instead of strncmp()/strlen() in
  hmc5843_check_sampling_frequency() (Lars-Peter Clausen)

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>