When an alias for a clock already exists the warning is printed. For
every module with a main_clk defined, a clk alias for fck is added.
There are some components that have the same main_clk defined, so this
is a really normal situation.

For example the am33xx edma device has 4 components using the same main
clock. So there are three warnings in the boot log for this already
existing clock alias:
	platform 49000000.edma: alias fck already exists
	platform 49000000.edma: alias fck already exists
	platform 49000000.edma: alias fck already exists

As this is only interesting for developers, this patch changes the
message to a debug message.
Signed-off-by: NMarkus Pargmann <mpa@pengutronix.de>
Signed-off-by: NTony Lindgren <tony@atomide.com>