[ARM] Orion: implement power-off method for QNAP TS-109/209

Since the PIC is attached to UART1, it doesn't need a kernel device driver
of its own; but powering off is something that the kernel should do, so
this patch forcefully configures the UART1 for 19200 baud and sends the
character that tells the PIC to cut the power.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Cc: Byron Bradley <byron.bbradley@gmail.com>
Acked-by: Nicolas Pitre <nico@marvell.com>