[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>

arch/arm/include/asm/kmap_types.h

@@ -22,4 +22,10 @@ enum km_type {
 	KM_TYPE_NR
 };
 
+#ifdef CONFIG_DEBUG_HIGHMEM
+#define KM_NMI		(-1)
+#define KM_NMI_PTE	(-1)
+#define KM_IRQ_PTE	(-1)
+#endif
+
 #endif