xen/arm: plat: Allocate as much as possible memory below 1GB for dom0 for RPI

xen/arm: plat: Allocate as much as possible memory below 1GB for dom0 for RPI

The raspberry PI 4 has devices that can only DMA into the first GB of
the RAM. Therefore we want allocate as much as possible memory below 1GB
for dom0.

Use the recently introduced dma_bitsize field to specify the DMA width
supported.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reported-by: Corey Minyard <minyard@acm.org>
Tested-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>