arm64: dts: imx8mm: Simplify mipi_dsi clocks

The device tree clock structure for the mipi_dsi is
unnecessarily redundant.

The default clock parent of IMX8MM_CLK_DSI_PHY_REF is
already IMX8MM_CLK_24M, so there is no need to set the
parent-child relationship between them.  The default clock
rates for IMX8MM_SYS_PLL1_266M and IMX8MM_CLK_24M are
already defined to be 266MHz and 24MHz respectively,
so there is no need to define those clock rates.

On i.MX8M[MNP] the  samsung,pll-clock-frequency is not
necessary, because the driver will read it from sclk_mipi
which is also already set to 24MHz making it also
redundant.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de> # Kontron BL i.MX8MM
Signed-off-by: Shawn Guo <shawnguo@kernel.org>