Recent Raspberry Pi firmware provides a mailbox property to detect
under-voltage conditions. Here is the current definition.

The u32 value returned by the firmware is divided into 2 parts:
  - lower 16-bits are the live value
  - upper 16-bits are the history or sticky value

  Bits:
  0: undervoltage
  1: arm frequency capped
  2: currently throttled
  16: undervoltage has occurred
  17: arm frequency capped has occurred
  18: throttling has occurred
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>