of: add 'const' for of_parse_phandle parameter *np

The existing function does not change the passed device_node pointer. It is
only handed to of_get_property which itself takes a const struct device_node.

of_parse_phandle() can therefore take a const pointer as well.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
[grant.likely: drop extraneous whitespace change]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>