xen/device_tree: introduce find_compatible_node

xen/device_tree: introduce find_compatible_node

Introduce a find_compatible_node function that can be used by device
drivers to find the node corresponding to their device in the device
tree.

Initialize device_tree_flattened early in start_xen, so that it is
available before setup_mm. Get rid of fdt in the process.

Also add device_tree_node_compatible to device_tree.h, that is currently
missing.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc - s/atag_paddr/fdt_paddr ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>