efi: Add 'systab' information to Documentation/ABI

It's not clear right now that the order in which entries are displayed
in /sys/firmware/efi/systab actually forms an ABI that userspace tools
rely upon.

Document the ABI along with the userspace tool that cares.
Suggested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Ivan Khoronzhuk <ivan.khoronzhuk@globallogic.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

Documentation/ABI/testing/sysfs-firmware-efi

@@ -18,3 +18,13 @@ Contact:	Dave Young <dyoung@redhat.com>
 Description:	It shows the physical address of config table entry in the EFI
 		system table.
 Users:		Kexec
+
+What:		/sys/firmware/efi/systab
+Date:		April 2005
+Contact:	linux-efi@vger.kernel.org
+Description:	Displays the physical addresses of all EFI Configuration
+		Tables found via the EFI System Table. The order in
+		which the tables are printed forms an ABI and newer
+		versions are always printed first, i.e. ACPI20 comes
+		before ACPI.
+Users:		dmidecode