xen/dts: Add new helpers to use the device tree

xen/dts: Add new helpers to use the device tree

List of new helpers taken from linux (commit 74b9272):
    - dt_property_read_string
    - dt_match_node
    - dt_find_maching_node
    - dt_device_is_available
    - dt_prop_cmp

Other new helpers:
    - dt_set_cell
    - dt_for_each_child
    - dt_set_range
    - dt_cells_to_size
    - dt_next_cell
    - dt_get_range
    - dt_node_name_is_equal
    - dt_node_path_is_equal
    - dt_property_name_is_equal

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>