panic: fix panic_timeout accuracy when running on a hypervisor

I've had some complaints about panic_timeout being wildly innacurate on
shared processor PowerPC partitions (a 3 minute panic_timeout taking 30
minutes).

The problem is we loop on mdelay(1) and with a 1ms in 10ms hypervisor
timeslice each of these will take 10ms (ie 10x) longer.  I expect other
platforms with shared processor hypervisors will see the same issue.

This patch keeps the old behaviour if we have a panic_blink (only keyboard
LEDs right now) and does 1 second mdelays if we don't.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>