rpi3: Add documentation of Raspberry Pi 3 port
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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 <antonio.ninodiaz@arm.com>
