iio: mlx90614: Add MLX90615 support

Add support for MLX90615 Infra Red Thermometer, which seems to be
the predecesor of MLX90614 . There are significant differences in
the register layout compared to MLX90614, but the functionality
of the device is virtually identical.

The following differences have been identified:
- RAM/EEPROM/SLEEP access opcodes are different
- RAM/EEPROM registers are at different offsets
- EEPROM emissivity and configuration registers are at different offsets
- EEPROM configuration register bits are shuffled around
- EEPROM emissivity settings are 14 bit on MLX90615 , 16 bit on MLX90614
- MLX90615 can only ever support one sensor, MLX90614 could support two
- FIR filter is set to fixed settings on MLX90615
- IIR filter coefficients are different

This patch fills in the MLX90615 specific description and quirk handling.

The IIR filter coefficients were provided by Melexis as follows:
0b0000 - Forbidden value
0b0001 - 100% - settling time = refresh rate = 205ms (around 5Hz)
0b0010 - 50% - settling time = 2050ms (around 0.5Hz)
0b0011 - 33% - settling time = 36900ms (around 0.3Hz)
0b0100 - 25% - settling time = 5125ms (around 0.2Hz)
0b0101 - 20% - settling time = 6355ms (around 0.15Hz)
0b0110 - 17% - settling time = 7790ms (around 0.13Hz)
0b0111 - 14% - settling time = 9225ms (around 0.1Hz)

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Crt Mori <cmo@melexis.com>
Link: https://lore.kernel.org/r/20230510102251.10118-6-marex@denx.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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