[POWERPC] Celleb: improve MMU hashtable locking

Disabling IRQ is required only in invalidation.  This changes
"spin_lock_irqsave" to "spin_lock" in other ops.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Akira Iguchi <akira2.iguchi@toshiba.co.jp>
Acked-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>