firmware/raspberrypi: Add a get_throttled sysfs file

Under-voltage due to inadequate power supplies is a recurring problem for
new Raspberry Pi users. There are visual indications that an
under-voltage situation is occuring like blinking power led and a
lightning icon on the desktop (not shown when using the vc4 driver), but
for new users it's not obvious that this signifies a critical situation.

This patch provides a twofold improvement to the situation:

Firstly it logs under-voltage events to the kernel log. This provides
information also for headless installations.

Secondly it provides a sysfs file to read the value. This improves on
'vcgencmd' by providing change notification. Userspace can poll on the
file and be notified of changes to the value.
A script can poll the file and use dbus notification to put a windows on
the desktop with information about the severity with a recommendation to
change the power supply. A link to more information can also be provided.
Only changes to the sticky bits are reported (cleared between readings).

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>