xen/arm: Print message if reset did not work

xen/arm: Print message if reset did not work

If for some reason the hardware reset is not working, print a message to
the user every 5 seconds to warn him that the system did not reset
properly and Xen is still looping.

The message is printed infinitely so that someone connecting to a serial
console with no history would see the message coming after 5 seconds.

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
[stefano: remove second '!']
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>