spapr: Add "qemu, boot-menu" property to /chosen

This is required to enable boot menu display during booting

Signed-off-by: Avik Sil <aviksil@linux.vnet.ibm.com>
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>