doc: arm: add UEFI support documentation

This patch provides documentation of the [U]EFI runtime service and
configuration features for the arm architecture.

Cc: Rob Landley <rob@landley.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Acked-by: Grant Likely <grant.likely@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
diff --git a/Documentation/arm/00-INDEX b/Documentation/arm/00-INDEX
index a94090c..3b08bc2 100644
--- a/Documentation/arm/00-INDEX
+++ b/Documentation/arm/00-INDEX
@@ -46,5 +46,7 @@
 	- SWP/SWPB emulation handler/logging description
 tcm.txt
 	- ARM Tightly Coupled Memory
+uefi.txt
+	- [U]EFI configuration and runtime services documentation
 vlocks.txt
 	- Voting locks, low-level mechanism relying on memory system atomic writes.