clk: Add API to get index of the clock parent

This patch adds a new clk_hw_get_parent_index() function that can be
used to retrieve the index of a given clock's parent. This can be useful
for restoring a clock on system resume.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>