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>