Platform/RPi4: Add EFI_MP_SERVICES_PROTOCOL support

Fix the ARM_MPCORE_INFO table and incorporate the DXE driver and test
app to the build so that EFI_MP_SERVICES_PROTOCOL can be used and tested
on Raspberry Pi 4. Note that the test app is not added to the image - it
can be taken from the build directory and executed from the UEFI shell.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Rebecca Cran <rebecca@quicinc.com>