i2c/sensors: Add driver for W83793 hardware monitor

The Winbond/Nuvoton W83793G system monitor chip includes many features
not currently supported by this driver.  The following are currently
supported:

* Up to 6 temperature sensors, 4 of which have 10-bit resolution
  (8.2),two with 8-bit resolution (no decimal component)
* Up to 12 fans
  - Fans 0-4 (1-5 on the datasheet) are always enabled.  The remaining 7
    fans are individually enabled.
* Multiple voltage sensors, reading up to 10 voltage sources.  Sysctls
  are labeled to match the datasheet.
* Chassis open detection.

The W83793AG is a feature-reduced version, which lacks 3 thermal diodes
and 2 voltage monitors.  Since there is no way to tell the difference
between the W83793AG and W83793G programmatically, sensors reported on
the W83793AG will report strange values.

Temperature sensors and 7 of the fans can be individually enabled on the
chip, but currently not configured by this driver.  The driver only
reports what was configured by the firmware.  Additionally, this driver
numbers the sensors and fans according to the datasheet, so even if, for
instance, fan 8 is disabled, it would skip from fan 7 to fan 9, it does
not renumber.  This makes it easier to follow for hardware debugging
purposes.

Missing features:
* Smart Fan support
* Fan PWM control
* ASF (Alert Standard Format)

Only Bank 0 registers are used at this time.

Reviewed by:	adrian
Differential Revision:	https://reviews.freebsd.org/D56776

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