ARM: dts: imx7ulp: remove mipi pll clock node

According to the IMX7ULP reference manual, the mipi pll
clock comes from the MIPI PHY PLL output. So it should
not be defined as a fixed clock. So remove this clock
node and all the references to it.

Signed-off-by: Fancy Fang <chen.fang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>