clk: fixed-factor: Let clk framework find parent

Convert this driver to a more modern way of specifying parents now that
we have a way to specify clk parents by DT index. This lets us nicely
avoid a problem where a parent clk name isn't know because the parent
clk hasn't been registered yet.

Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Jerome Brunet <jbrunet@baylibre.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Jeffrey Hugo <jhugo@codeaurora.org>
Cc: Chen-Yu Tsai <wens@csie.org>
Tested-by: Jeffrey Hugo <jhugo@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>