arm: dts: imx8mp: Delete default clocks for clock controller node

Since SPL has initialized clocks for bus and core. We don't need to
set the default clocks for clock controller node.

Signed-off-by: Ye Li <ye.li@nxp.com>
Tested-by: Teresa Remmet <t.remmet@phytec.de>
Tested-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Acked-by: Peng Fan <peng.fan@nxp.com>