[PATCH] powerpc: work around a cell interrupt HW bug

Apparently we have found a bug in the CPU that causes
external interrupts to sometimes get disabled indefinitely.
This adds a workaround for the problem.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>