xen/arm: setup: nr_banks should be unsigned int

xen/arm: setup: nr_banks should be unsigned int

It is not possible to have a negative number of banks. So switch to
unsigned int.

The type change is also propagated to any users of nr_banks that were
using "int" (there are not that many).

Note that fdt_num_mem_rsv() can actually returns a negative value in
case of an error. So the return should be checked before assigning the
result to an unsigned variable.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Wei Chen <Wei.Chen@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>