xen/arm: Take into account the DMA width when allocating Dom0 memory banks

xen/arm: Take into account the DMA width when allocating Dom0 memory banks

At the moment, Xen is assuming that all the devices are at least 32-bit
DMA capable. However, some SoCs have devices that may be able to access
a much restricted range. For instance, the Raspberry PI 4 has devices
that can only access the first GB of RAM.

The function arch_get_dma_bit_size() will return the lowest DMA width on
the platform. Use it to decide what is the limit for the low memory.

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