Platform/RaspberryPi4: Clean up PCDs out of the GENET driver

Move PCDs from GENET driver to Raspberry Pi and Bcm27xx packages.
The Genet driver follows the UEFI driver model, so it should not have
PCDs defined that describe MMIO and MAC addresses of a single instance.
Also, move related definitions around, and update references accordingly.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>