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 <[e-mail]>
Acked-by: Ard Biesheuvel <[e-mail]>
Reviewed-by: Jean Delvare <[e-mail]>
Cc: Leif Lindholm <[e-mail]>
Cc: Ivan Khoronzhuk <[e-mail]>
Signed-off-by: Matt Fleming <[e-mail]>

Documentation/ABI/testing/sysfs-firmware-efi

···
 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:	[e-mail]
+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