ARM: dt: Only print warning, not WARN() on bad cpu map in device tree

Due to recent changes and expecations of proper cpu bindings, there are
now cases for many of the in-tree devicetrees where a WARN() will hit
on boot due to badly formatted /cpus nodes.

Downgrade this to a pr_warn() to be less alarmist, since it's not a
new problem.

Tested on Arndale, Cubox, Seaboard and Panda ES. Panda hits the WARN
without this, the others do not.

Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>