clk: Allow parents to be specified via clkspec index

Some clk providers are simple DT nodes that only have a 'clocks'
property without having an associated 'clock-names' property. In these
cases, we want to let these clk providers point to their parent clks
without having to dereference the 'clocks' property at probe time to
figure out the parent's globally unique clk name. Let's add an 'index'
property to the parent_data structure so that clk providers can indicate
that their parent is a particular index in the 'clocks' DT property.

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>