arm64: dts: imx8mn-evk: enable uart1

Enable uart1 for BT usage
Configure the clock to source from IMX8MN_SYS_PLL1_80M, because the uart
could only support max 1.5M buadrate if using OSC_24M as clock source.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>