clk: fixed-factor: Introduce devm_clk_hw_register_fixed_factor_index()

Add an API for a fixed factor clk that uses an index for the parent
instead of a string name. This allows us to move drivers away from the
string based method of describing parents and use the DT/firmware based
method instead.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: devicetree@vger.kernel.org
Link: https://lore.kernel.org/r/20220226040723.143705-2-marex@denx.de


[sboyd@kernel.org: Expose a new API instead of internal function]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>