powerpc: Prepare Book-E kernels for KERNBASE != run base

Book-E kernels really run at VM_MIN_KERNEL_ADDRESS, which currently happens to
be the same as KERNBASE. KERNBASE is the linked address, which the loader also
takes to be the physical load address. Treat KERNBASE as a physical address,
not a virtual, and change virtual address references for KERNBASE to use
something more appropriate.