arm64: dts: imx8: update lpuart[0..3] irq number

Original irq number combined UART irq and DMA irq. These doesn't match
uart driver and dma engine's expection.

Update to the irq numbers, which just uart can trigger.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>