Platform/RPi/AcpiTables: Clean up ACPI IDs

Only apply the "BC2836" override to the one table that needs it for
Pi 3, instead of setting it globally.
Use more appropriate OEM and Vendor IDs for hardware that is
produced by the Raspberry Pi Foundation.
Ensure that the Pi 3 and Pi 4 have different OEM IDs.
Additionally fix a comment typo (MEMORY32SETBASE -> MEMORY32FIXED).

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