ARM: dts: imx: drop 'clocks' container for board level clocks

For soc level of clocks which are input to Clock Control Module, clock
driver expects them to be under 'clocks' container node.  But for board
level clocks, this container is not really necessary.  Let's drop it and
use an unique name for fixed rate clock, so that 'reg' property can be
saved as well.

Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Acked-By: Lothar Waßmann <LW@KARO-electronics.de>