Platform/RPi: Fix Readme issues

* device_tree_address/end was incorrect for RPi3.
* Link to TF-A Readme.md in non-osi was broken.
* RPi4 now uses TF-A binaries built from official source.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>