[POWERPC] PS3: os-area workqueue processing

Add a workqueue to the PS3 os-area support.  This is needed to
support writing updates to flash memory and to update the /proc
device tree entries from the timer tick interrupt context.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>