[POWERPC] ps3: get firmware version

Add a new routine ps3_get_firmware_version() and use it to output the firmware
version to dmesg.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>