ARM: dts: ls1021a: update the clockgen node

qoriq clock driver has been updated to parse the clock configuration
information defined in driver itself not in dts.
Since the new implementation and the bindings have been merged,
it is time to update the clock related node and remove redundent clock
configuration information from the dts.

Signed-off-by: Tang Yuantian <andy.tang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>