MLK-20718-3: arm64: dts: imx8dx: Use DSI PHY_REF clk

MLK-20718-3: arm64: dts: imx8dx: Use DSI PHY_REF clk

Until now, the DSI PHY_REF clock was by default ON in SCFW, which made
this clock unusable in kernel, therefore, this clock was set as
CLK_DUMMY in DSI device nodes.
Sinnce this clock was set to OFF in SCFW, now it can be used from
kernel, so add it to device nodes so that the driver can use it
properly.

Signed-off-by: Robert Chiras <robert.chiras@nxp.com>
Reviewed-by: Laurentiu Palcu <laurentiu.palcu@nxp.com>