[POWERPC] PS3: Use the HVs storage device notification mechanism properly

The PS3 hypervisor has a storage device notification mechanism to wait
until a storage device is ready.  Unfortunately the storage device
probing code used this mechanism in an incorrect way, needing a
polling loop and handling of devices that are not yet ready.

This change corrects this by:
  - First waiting for the reception of an asynchronous notification
    that a new storage device became ready,
  - Then looking up the storage device in the device repository.

On shutdown, the storage probe thread is stopped and the storage
notification device is closed using a reboot notifier.

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