rpi3: Add documentation of Raspberry Pi 3 port
rpi3: Add documentation of Raspberry Pi 3 port

Added design documentation and usage guide for the AArch64 port of the
Arm Trusted Firmware to the Raspberry Pi 3.

Change-Id: I1be60fbbd54c797b48a1bcebfb944d332616a0de
Signed-off-by: Antonio Nino Diaz <5c24ff39be768bf31f4deee44042a11772cfd02a@arm.com>

lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public