iio: temperature: maxim_thermocouple: declare missing of table

Add missing <of_device_id> table for SPI driver relying on SPI
device match since compatible is in a DT binding or in a DTS.

Before this patch:
modinfo drivers/iio/temperature/maxim_thermocouple.ko | grep alias
alias:          spi:max31855
alias:          spi:max6675

After this patch:
modinfo drivers/iio/temperature/maxim_thermocouple.ko | grep alias
alias:          spi:max31855
alias:          spi:max6675
alias:          of:N*T*Cmaxim,max31855C*
alias:          of:N*T*Cmaxim,max31855
alias:          of:N*T*Cmaxim,max6675C*
alias:          of:N*T*Cmaxim,max6675

Reported-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Daniel Gomez <dagmcr@gmail.com>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>