ARM: dts: imx7ulp: update the LPI2C clock-names

The clock name should be per clock, not ipg clock.

Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>