Fix detection of memory overlap with the kernel in the case where a memory
region marked "available" by firmware is contained entirely in the kernel.

This had a tendency to happen with FDTs passed by loader, though could for
other reasons as well, and would result in the kernel slowly cannibalizing
itself for other purposes, eventually resulting in a crash.

A similar fix is needed for mmu_oea.c and should probably just be rolled
at that point into some generic code in platform.c for taking a mem_region
list and removing chunks.

PR: 226974
Submitted by: leandro.lupori@gmail.com
Reviewed by: jhibbits
Differential Revision: D15121