morello: Add Morello platform documentation

Morello platform has a SCP which brings the primary Rainier CPU
out of reset which starts executing at BL31.

This patch provides documentation support for Morello platform.
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>
Change-Id: I38f596668e2b14862d543fabc04549ff34bfb8a2

docs/plat/arm/index.rst

@@ -11,6 +11,7 @@ Arm Development Platforms
    tc0/index
    arm_fpga/index
    arm-build-options
+   morello/index
 
 This chapter holds documentation related to Arm's development platforms,
 including both software models (FVPs) and hardware development boards

docs/plat/arm/morello/index.rst

+Morello Platform
+================
+
+Morello is an ARMv8-A platform that implements the capability architecture extension.
+The platform port present at `site <https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git>`_
+provides ARMv8-A architecture enablement.
+
+Capability architecture specific changes will be added `here <https://git.morello-project.org/morello>`_
+
+Further information on Morello Platform is available at `info <https://developer.arm.com/architectures/cpu-architecture/a-profile/morello>`_
+
+Boot Sequence
+-------------
+
+The execution begins from SCP_BL1 which loads the SCP_BL2 and starts its
+execution. SCP_BL2 powers up the AP which starts execution at AP_BL31. The AP
+then continues executing and hands off execution to Non-secure world (UEFI).
+
+Build Procedure (TF-A only)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+-  Obtain arm `toolchain <https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-a/downloads>`_.
+   Set the CROSS_COMPILE environment variable to point to the toolchain folder.
+
+-  Build TF-A:
+
+   .. code:: shell
+
+      export CROSS_COMPILE=<path-to-aarch64-gcc>/bin/aarch64-none-elf-
+
+      make PLAT=morello all
+
+*Copyright (c) 2020, Arm Limited. All rights reserved.*