powerpc64: Avoid overwriting initrd area

Currently kexec loads an initrd file into the main memory but does not
mark that region as reserved, thus the area is not protected.

If any initrd/md file is loaded from kexec/petitboot, the region might become
corarupted/overwritten since FreeBSD does not know the region is 'reserved'.

This patch simply adds the initrd area as a reserved memory region.

Approved by: jhibbits
Differential Revision: https://reviews.freebsd.org/D15610