Platform/RaspberryPi4: set DMA translation for BCM Genet driver

The driver for the Broadcom Genet network controller will shortly
be modified from a minimal MAC address programming driver to a true
SNP driver implementing full network functionality. Since this will
involve DMA, set the DmaLib parameters correctly in the platform.

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