Platform/RPi4: Clean up ACPI definitions

* Use ACPI 5.1 everywhere, since we are constrained to use v5.x for
  MADT compatibility.
* Clean up whitespaces and reorganize header declaration.
* Prefix all RPi related constant with RPI_ to make them clearer to
  differentiate from regular EDK2 ones.
* Reference IndustryStandard/Acpi.h always.
* Remove explicit references to RPI4 for sources that we may be
  factorized for both the Pi 3 and Pi 4 platform.

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