xen/arm: domain_build: Use copy_to_guest_phys_flush_dcache in dtb_load

xen/arm: domain_build: Use copy_to_guest_phys_flush_dcache in dtb_load

The function dtb_load is dealing with IPA but uses gvirt_to_maddr to do
the translation. This is currently working fine because the stage-1 MMU
is disabled.

Rather than relying on such assumption, use the new
copy_to_guest_phys_flush_dcache. This also result to a slightly more
comprehensible code.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>