spapr: Respect -bios command line option for SLOF

Allow the user to override the firmware file name rather than always
using "slof.bin".

Reported-by: Dinar Valeev <k0da@opensuse.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
