drivers/sensor: si7006: Support SHT21 and HTU21D

These three sensor types are all largely compatible.  The SHT21 and
HTU21D can be supported by this driver by sending command 0xE3 instead
of 0xE0 to read the temperature.

Mention the sensor names in bindings and Kconfig to help those looking
for support to find it.  There have been at least five PRs attempting to
add SHT21 and/or HTU21D support that did not realize the Si7006 is the
same.

As mentioned in PR #22862, the Sensirion SH21 is the original.  The dts
bindings are adjusted (in a backward compatible way!) to make the sht21
the base binding and si7006 is derived from that.

Examples of dts compatibles:

TE Connectivity née Measurement Sepcialties HTU21D:
compatible = "meas,htu21d", "sensirion,sht21";

Sensirion SHT21:
compatible = "sensirion,sht21";

Silicon Labs Si7006
compatible = "silabs,si7006";

Silicon Labs Si7021
compatible = "silabs,si7021", "silabs,si7006";

Signed-off-by: Trent Piepho <tpiepho@gmail.com>