drivers: clk: Fix using assigned-clocks in the node of the clock it sets up

drivers: clk: Fix using assigned-clocks in the node of the clock it sets up

This fixes the case where assigned-clocks is used to define a clock
defaults inside this same clock's node. This is used sometimes to setup a
default parents and/or rate for a clock.

example:
muxed_clock: muxed_clock {
	clocks = <&clk_provider 0>, <&clk_provider 1>;
	#clock-cells = <0>;
	assigned-clocks = <&muxed_clock>;
	assigned-clock-parents = <&clk_provider 1>;
};

It doesn't work in u-boot because the assigned-clocks are setup *before*
the clock is probed. (clk_set_parent() will likely crash or fail if called
before the device probe function)
Making it work by handling "assigned-clocks" in 2 steps: first before the
clk device is probed, and then after the clk device is probed.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>