The fixmap code from x86 allows us to have compile time virtual addresses
that we change the physical addresses of at run time.

This is useful for applications like kmap_atomic, PCI config that is done
via direct memory map, kexec/kdump.

We got ride of CONFIG_HIGHMEM_START as we can now determine a more optimal
location for PKMAP_BASE based on where the fixmap addresses start and
working back from there.

Additionally, the kmap code in asm-powerpc/highmem.h always had debug
enabled.  Moved to using CONFIG_DEBUG_HIGHMEM to determine if we should
have the extra debug checking.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>