hwmon: Add support for RPi voltage sensor

Currently there is no easy way to detect undervoltage conditions on a
remote Raspberry Pi. This hwmon driver retrieves the state of the
undervoltage sensor via mailbox interface. The handling based on
Noralf's modifications to the downstream firmware driver. In case of
an undervoltage condition only an entry is written to the kernel log.

CC: "Noralf Trønnes" <noralf@tronnes.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Guenter Roeck <linux@roeck-us.net>