fdt_support: Add some useful functions

Add functions
  fdt_node_offset_by_pathf(),
  fdt_create_phandle_by_pathf(),
  fdt_set_status_by_pathf()
to get node offset, get/create node phandle and set status for node
given by path/alias formatted with sprintf.

Add functions
  fdt_create_phandle_by_compatible(),
  fdt_set_status_by_compatible()
to get/create node phandle and set status for first node given by
compatible.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>