reset: Silence warning in reset-controller.h

If a user of <linux/reset-controller.h> doesn't include
<linux/of.h> before including reset-controller.h they'll get a
warning as follows:

  include/linux/reset-controller.h:44:17:
  warning: 'struct of_phandle_args' declared inside parameter list

This is because of_phandle_args is not forward declared. Add the
declaration to silence this warning.

Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>