Update Arm TF references to TF-A

Update Arm TF references to TF-A

Update Arm Trusted Firmware references in the upstream documents to
Trusted Firmware-A (TF-A). This is for consistency with and
disambiguation from Trusted Firmware-M (TF-M).

Also update other Arm trademarks, e.g. ARM->Arm, ARMv8->Armv8-A.

Change-Id: I8bb0e18af29c6744eeea2dc6c08f2c10b20ede22
Signed-off-by: Dan Handley <[email protected]>
Signed-off-by: David Cunado <[email protected]>