xen/arm: add iounmap after initrd has been loaded in domain_build

xen/arm: add iounmap after initrd has been loaded in domain_build

domain_build use ioremap_wc to map a new non-cacheable virtual
address for initrd. After Xen copy initrd from this address to
guest, this new allocated virtual address has not been unmapped.

So in this patch, we add an iounmap to the end of domain_build,
after Xen loaded initrd to guest memory.

Signed-off-by: Wei Chen <wei.chen@arm.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>