x86: Move table csum into separate file

We need the checksum function without all the other table functionality
soon, so let's split it out into its own C file.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/include/asm/tables.h

@@ -7,6 +7,8 @@
 #ifndef _X86_TABLES_H_
 #define _X86_TABLES_H_
 
+#include <tables_csum.h>
+
 /*
  * All x86 tables happen to like the address range from 0xf0000 to 0x100000.
  * We use 0xf0000 as the starting address to store those tables, including