arm64: dts: imx8mq: Add clock parents for mipi dphy

This makes sure the clock tree setup for the dphy is not dependent on
other components.

Without this change bringing up the display can fail like

  kernel: phy phy-30a00300.dphy.2: Invalid CM/CN/CO values: 165/217/1
  kernel: phy phy-30a00300.dphy.2: for hs_clk/ref_clk=451656000/593999998 ~ 165/217

if LCDIF doesn't set up that part of the clock tree first. This was
noticed when testing the Librem 5 devkit with defconfig. It doesn't
happen when modules are built in.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>