Platform/RPi4: Update DSC to reflect switch to single TF-A binary

Note: This commit depends on an "Update TF-A binaries to v2.3" patch
having been applied to edk2-non-osi.

With the update of non-osi TF-A binaries to latest, we can switch to
using the single bl31.bin instead of the now removed bl31_pl011.bin.

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