clk: imx7ulp: do not export out IMX7ULP_CLK_MIPI_PLL clock

The mipi pll clock comes from the MIPI PHY PLL output, so
it should not be a fixed clock.

MIPI PHY PLL is in the MIPI DSI space, and it is used as
the bit clock for transferring the pixel data out and its
output clock is configured according to the display mode.

So it should be used only for MIPI DSI and not be exported
out for other usages.

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