reset: add of_reset_control_get_by_index()

Add of_reset_control_get_by_index() to allow the drivers to get reset
device without knowing its name.

Signed-off-by: Vince Hsu <vinceh@nvidia.com>
[jonathanh@nvidia.com: Updated stub function to return -ENOTSUPP instead
 of -ENOSYS which should only be used for system calls.]
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>