trusty: Add boot parameter documentation

Change-Id: Ibfb75145e3a31ae2106eedfbe4a91c2e31bb9f2a

docs/spd/trusty-dispatcher.rst

@@ -8,6 +8,23 @@ Detailed information and build instructions can be found on the Android
 Open Source Project (AOSP) webpage for Trusty hosted at
 https://source.android.com/security/trusty
 
+Boot parameters
+===============
+
+Custom boot parameters can be passed to Trusty by providing a platform
+specific function:
+
+.. code:: c
+
+    void plat_trusty_set_boot_args(aapcs64_params_t *args)
+
+If this function is provided ``args->arg0`` must be set to the memory
+size allocated to trusty. If the platform does not provide this
+function, but defines ``TSP_SEC_MEM_SIZE``, a default implementation
+will pass the memory size from ``TSP_SEC_MEM_SIZE``. ``args->arg1``
+can be set to a platform specific parameter block, and ``args->arg2``
+should then be set to the size of that block.
+
 Supported platforms
 ===================