arm64: Add FIX_HOLE to permanent fixed addresses

Every other architecture with permanent fixed addresses has
FIX_HOLE as the first entry. This seems to be designed as a
debugging aid but there are a couple of side effects of not
having FIX_HOLE:

- If the first fixed address is 0, fix_to_virt -> virt_to_fix
triggers a BUG_ON for the virtual address being equal to
FIXADDR_TOP
- fix_to_virt may return a value outside of FIXADDR_START
and FIXADDR_TOP which may look like a bug to a developer.

Match up with other architectures and make everything clearer
by adding FIX_HOLE.
Signed-off-by: NLaura Abbott <lauraa@codeaurora.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/include/asm/fixmap.h

@@ -31,6 +31,7 @@
  *
  */
 enum fixed_addresses {
+	FIX_HOLE,
 	FIX_EARLYCON_MEM_BASE,
 	__end_of_permanent_fixed_addresses,