powerpc: Clean up panic_timeout usage

powerpc: Clean up panic_timeout usage

Default CONFIG_PANIC_TIMEOUT to 180 seconds on powerpc. The
pSeries continue to set the timeout to 10 seconds at run-time.

Thus, there's a small window where we don't have the correct
value on pSeries, but if this is only run-time discoverable we
don't have a better option. In any case, if the user changes the
default setting of 180 seconds, we honor that user setting.

Signed-off-by: Jason Baron <jbaron@akamai.com>
Cc: benh@kernel.crashing.org
Cc: paulus@samba.org
Cc: ralf@linux-mips.org
Cc: mpe@ellerman.id.au
Cc: felipe.contreras@gmail.com
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/705bbe0f70fb20759151642ba0176a6414ec9f7a.1385418410.git.jbaron@akamai.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>