clk: versatile: Switch to assigned clock parents

We're removing struct clk from the clk provider API. This code is
calling the consumer APIs to change the parent to a 1 MHz fixed
rate clock for each of the clocks that the driver provides. Move
to using the assigned-clock-parents DT property for this instead.
Because this is an ABI break, detect if the property is missing
and fall back to setting the parent explicitly before the clocks
are registered.

Acked-by: Pawel Moll <pawel.moll@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>