doc: brcm: Add documentation file for brcm stingray platform

Signed-off-by: Sheetal Tigadoli <sheetal.tigadoli@broadcom.com>
Change-Id: I5e2c1220e9694d6ba771cc90daa0e70e967eebe6

docs/plat/brcm-stingray.rst

+Description
+===========
+Broadcom's Stingray(BCM958742t) is a multi-core processor with 8 Cortex-A72 cores.
+Trusted Firmware-A (TF-A) is used to implement secure world firmware, supporting
+BL2 and BL31 for Broadcom Stingray SoCs
+
+On Poweron, Boot ROM will load bl2 image and Bl2 will initialize the hardware,
+then loads bl31 and bl33 into DDR and boots to bl33.
+
+Boot Sequence
+=============
+
+Bootrom --> TF-A BL2 --> TF-A BL31 --> BL33(u-boot)
+
+Code Locations
+--------------
+-  Trusted Firmware-A:
+   `link <https://github.com/ARM-software/arm-trusted-firmware>`__
+
+How to build
+============
+
+Build Procedure
+---------------
+
+-  Prepare AARCH64 toolchain.
+
+-  Build u-boot first, and get the binary image: u-boot.bin,
+
+-  Build TF-A
+
+   Build fip:
+
+   .. code::shell
+
+       make CROSS_COMPILE=aarch64-linux-gnu- PLAT=stingray BOARD_CFG=bcm958742t all fip BL33=u-boot.bin
+
+Deploy TF-A Images
+-----------------
+The u-boot will be upstreamed soon, this doc will be updated once they are ready, and the link will be posted.