dts: xtensa: nxp_imx8: Add dummy interrupt controller node

Since the LPUART peripheral DTS binding requires the
"interrupts" property be specified even if it's not going
to be used for now we need to add a dummy interrupt controller
node to make that possible. Logically speaking, this dummy
interrupt controller should be used by peripherals which
can assert interrupts directly routed to the DSP.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>