add 'poweroff' support

[Antony Pavlov]

This patch series adds the ability to turn power off on
Ritmix RZX-50 game console.

The Ritmix RZX-50 game console is a battery-powered device.
So I you keep it running barebox it can to use up the battery
in 14-16 hours. So it is very desirable to be able to turn
power off from barebox.

[PATCH 1/2] add 'poweroff' command
[PATCH 2/2] rzx50: add poweroff support