[POWERPC] PS3: Add ps3_repository_find_device_by_id()

The storage probe feature of the PS3 hypervisor returns device IDs.  Add
the corresponding repository routine ps3_repository_find_device_by_id()
which can be used to retrieve the device info from the repository.

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>