allwinner: Add BL32 (corresponds to Trusted OS) support

This patch is an attempt to run Trusted OS (OP-TEE OS being one of them) along
side BL31 image.

ATF supports multiple SPD's that can take dispatcher name (opteed for OP-TEE OS)
as an input using the 'SPD=<dispatcher name>' option during bl31 build.

Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>
diff --git a/docs/plat/allwinner.rst b/docs/plat/allwinner.rst
index a7e84a3..a10e6dc 100644
--- a/docs/plat/allwinner.rst
+++ b/docs/plat/allwinner.rst
@@ -27,3 +27,13 @@
     make CROSS_COMPILE=aarch64-linux-gnu- PLAT=sun50i_a64 DEBUG=1 bl31
 
 .. _U-Boot documentation: http://git.denx.de/?p=u-boot.git;f=board/sunxi/README.sunxi64;hb=HEAD
+
+Trusted OS dispatcher
+=====================
+
+One can boot Trusted OS(OP-TEE OS, bl32 image) along side bl31 image on Allwinner A64.
+
+In order to include the 'opteed' dispatcher in the image, pass 'SPD=opteed' on the command line
+while compiling the bl31 image and make sure the loader (SPL) loads the Trusted OS binary to
+the beginning of DRAM (0x40000000).
+