Platform/RaspberryPi: Increase genet dma window

The genet is capable of addressing the entire memory space
on the RPI4. Lets allow it to dma into those regions.
This solves intermittent issues with grub/etc being able
to communicate when the 3G limit is lifted on 8G boards.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Jared McNeill <jmcneill@invisible.ca>