MFC r350591, r350971, r351724

r350591:
Add a driver for Texas Instruments ADS101x/ADS111x i2c ADC chips.

Instances of the device can be configured using hints or FDT data.

Interfaces to reconfigure the chip and extract voltage measurements from
it are available via sysctl(8).

r350971:
Fix the driver name in ads111x.4, and hook the manpage up to the build.

The driver was originally written with the name ads1115, but at the last
minute it got renamed to ads111x to reflect its support for many related
chips, but I forgot to update the manpage to match the renaming before
committing it all.

r351724:
Fix the name of the devicetree bindings document file cited in the manpage.