of: *node argument to of_parse_phandle_with_args should be const

The "struct device_node *" argument of of_parse_phandle_with_args() can
be const. Making this change makes it explicit that the function will
not modify a node.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
[grant.likely: Resolved conflict with previous patch modifying of_parse_phandle()]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>