pseries: set no default boot order

This patch removes the default boot order for pseries machine. This allows
the machine to handle a NULL boot order in case no -boot option is provided.
Thus it helps SLOF firmware to verify if boot order is specified in command
line or not. If no boot order is provided SLOF tries to boot from the
device set in the nvram.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avik Sil <aviksil@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
