iio: humidity: si7020: add heater support

This patch adds support for the integrated on-chip heater that is present
on all the devices supported by this driver (si7020, si7021, si7013, th6).
In order to configure the heater, the driver interacts with the following
device registers:
* User Register - the 2nd bit of this register is a "Heater Enable bit"
  (0 means that the heater is off, 1 means that it's on).
* Heater Register - this register is present only on the si70xx devices
  and controls the current flowing through the heater. The 4 lower bits
  of this register can be assigned values from 0x0 to 0xF.

Signed-off-by: Antoni Pokusinski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Cameron <[email protected]>

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