clk: imx: vf610: fix DCU clock tree

Similar to an earlier fix for the SAI clocks, the DCU clock hierarchy
mixes the bus clock with the display controllers pixel clock. Tests
have shown that the gates in CCM_CCGR3/9 registers do not control
the DCU pixel clock, but only the register access clock (bus clock).

Fix this by defining the parent clock of VF610_CLK_DCUx to be the bus
clock (ipg_bus).

Since the clock has not been used far, there are no further changes
needed.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>