Incorrect use of 0 in terminating entry of arch_tables[] causes the
following sparse warning,

  arch/x86/platform/efi/efi.c:74:27: sparse: Using plain integer as NULL pointer

Replace with NULL.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
[ Included sparse warning in commit message. ]
Signed-off-by: Matt Fleming <matt.fleming@intel.com>