iio: maxim_thermocouple: add thermocouple_type sysfs attribute

iio: maxim_thermocouple: add thermocouple_type sysfs attribute

We added a sysfs ABI for getting/setting the type of a thermocouple. This
driver supports chips that support specific fixed thermocouple types; we
cannot set it, but still we can add this sysfs attribute in RO mode to
read-back the thermocouple type.

This driver supports actually several chips:
 - max6675
 - max31855[k/j/n/s/t/e/r]asa family

Max6675 supports only K-type thermocouples, so we can just report that.

Each chip in max31855 family supports just one specific thermocouple type
(in the obvious way: i.e. max31855jasa supports J-type). This driver did
accept a generic SPI ID and OF compatible "max31855" which does not give
any clue about which chip is really involved (and unfortunately it seems
we have no way to detect it).

This patch introduces a new set of, more specific, SPI IDs and OF
compatible strings to better match the chip type.

The old, generic, "max31855" binding is kept for compatibility reasons, but
this patch aims to deprecate it, so, should we hit it, a warning is spit.
In such case the reported thermocouple type in sysfs is '?', because we
have no way to know.

Regarding the implementation: the thermocouple type information is stored
in the driver private data and I've kept only two maxim_thermocouple_chip
types in order to avoid a lot of duplications (seven chip types with just
a different thermocouple type).

RFT because I have no real HW to test this.

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
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>