powerpc64: make autoboot possible on powernv machines

It's required to specify a default boot option in order to make
petitboot's autoboot feature work.

Tested on Raptor Blackbird

Reviewed by: imp, luporl
MFC after: 2 days
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D32838