The ads784x chips include not just touchscreen sensors but also temperature
and voltage sensors, which are traditional hwmon fodder.

 - Register with the hwmon framework
    * hwmon needs to use subsys_init() so it's available early enough
 - Adopt hwmon convention for presenting voltages:
    * Report voltages in millivolts
    * Attribute names are "in[0-8]_input" (meaningless, ugh)
 - Be more accurate for the hwmon values:
    * vBATT gets range-adjusted
    * Discard the extra bit sent before the sample.

Battery measurements help during battery recharge monitoring.  On OSK/Mistral,
the measured voltage agreed with a multimeter until the fourth decimal place.

NOTE:  this version is only for the OMAP tree, which has some byteswap bugs
(and other changes) that are not found in the upstream kernels.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>