powerpc: Fix DEBUG_HIGHMEM build break from d4515646699

Code was added to mm/higmem.c that depends on several
kmap types that powerpc does not support.  We add dummy
invalid definitions for KM_NMI, KM_NM_PTE, and KM_IRQ_PTE.

According to list discussion, this fix should not be needed
anymore starting with 2.6.33.  The code is commented to this
effect so hopefully we will remember to remove this.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>