xen: move declarations of device_tree_get_{reg,u32}() to xen/device_tree.h

The definitions of device_tree_get_reg() and device_tree_get_u32() are already
in common code, so move their prototypes there as well.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>