Platform/RPi4: Improve FADT ACPI table generation

Use a proper aslc source to build the table.

Note that we use ACPI 5.1 for this table to match the MADT
constraints.

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