kboot: Add powerpc stub for enumerate_memory_arch()

Add stub for new MI interface for enumerating memory. Right now powerpc
looks in the FDT table at a later point in boot since we don't need to
pass a specific memory table to the kernel. Leave it like that for now,
but note plans for the future.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38248