clk: Add clk_parent entry in debugfs

This allows to easily determine the parent in shell scripts without
parsing more complex files.

Add the clk_parent file for all clks which can have a parent, not just
muxes. This way it can be used to determine the clk tree structure
without parsing more complex files.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>