cli: Increase reboot timeout to 15 mins

I've had reports of a particular server system taking longer than 10
mins to reboot and the reboot timeout kicking in. As a quick fix, let's
increase the timeout to 15 mins. A better solution would be to make this
programmable via the plan. We can implement that if 15 mins proves to be
too short for some.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>