clk: Fix debugfs clk_possible_parents for clks without parent string names

clk: Fix debugfs clk_possible_parents for clks without parent string names

Following the commit fc0c209c147f ("clk: Allow parents to be specified
without string names"), the parent name string is not always populated.

Instead, fetch the parents clk_core struct using the appropriate helper,
and read its name directly. If that fails, go through the possible
sources of parent names. The order in which they are used is different
from how parents are looked up, with the global name having precedence
over local fw_name and indices. This makes more sense as a) the
parent_maps structure does not differentiate between legacy global names
and fallback global names, and b) global names likely provide more
information than local fw_names.

Fixes: fc0c209c147f ("clk: Allow parents to be specified without string names")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>