x86/efi: Fix config_table_type array termination

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

arch/x86/platform/efi/efi.c

@@ -71,7 +71,7 @@ static __initdata efi_config_table_type_t arch_tables[] = {
 #ifdef CONFIG_X86_UV
 	{UV_SYSTEM_TABLE_GUID, "UVsystab", &efi.uv_systab},
 #endif
-	{NULL_GUID, NULL, 0},
+	{NULL_GUID, NULL, NULL},
 };
 
 /*