Add the assigned-clocks and assigned-clock-rates properties for the
LPUARTx nodes. Without these properties, the default clock rate
used would be 0, which can cause the UART ports to fail when open.

Fixes: 35f4e9d7 ("arm64: dts: imx8: split adma ss into dma and audio ss")
Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>