Make options MD_ROOT_MEM default on PPC64

Having this option enabled by default on PowerPC64 kernels makes
booting ISO images much easier when on PowerNV.

With it, the ISO may simply be given to the -i flag of kexec.
Better yet, the ISO may be loop mounted on PetitBoot and its
kernel may be used to load itself.

Without this option, booting ISOs on remote PPC64 machines usually
involve preparing a separate kernel, with this option enabled.