[ARM] kmap: fix build errors with DEBUG_HIGHMEM enabled

d4515646 broke ARM by requiring KM_IRQ_PTE, KM_NMI and KM_NMI_PTE to
always be defined.  Solve this by providing invalid definitions for
these constants, but only if CONFIG_DEBUG_HIGHMEM is enabled.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>