imx: imx8mn_evk: enable CONFIG_DM_SERIAL

Enable CONFIG_DM_SERIAL. uart2 and its pinmux was already
marked with u-boot,dm-spl.

move the preloader_console_init() call after spl_init() to
avoid board hang

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/board/freescale/imx8mn_evk/spl.c b/board/freescale/imx8mn_evk/spl.c
index dfa81a0..63f335b 100644
--- a/board/freescale/imx8mn_evk/spl.c
+++ b/board/freescale/imx8mn_evk/spl.c
@@ -115,14 +115,8 @@
 }
 #endif
 
-#define UART_PAD_CTRL	(PAD_CTL_DSE6 | PAD_CTL_FSEL1)
 #define WDOG_PAD_CTRL	(PAD_CTL_DSE6 | PAD_CTL_ODE | PAD_CTL_PUE | PAD_CTL_PE)
 
-static iomux_v3_cfg_t const uart_pads[] = {
-	IMX8MN_PAD_UART2_RXD__UART2_DCE_RX | MUX_PAD_CTRL(UART_PAD_CTRL),
-	IMX8MN_PAD_UART2_TXD__UART2_DCE_TX | MUX_PAD_CTRL(UART_PAD_CTRL),
-};
-
 static iomux_v3_cfg_t const wdog_pads[] = {
 	IMX8MN_PAD_GPIO1_IO02__WDOG1_WDOG_B  | MUX_PAD_CTRL(WDOG_PAD_CTRL),
 };
@@ -135,8 +129,6 @@
 
 	set_wdog_reset(wdog);
 
-	imx_iomux_v3_setup_multiple_pads(uart_pads, ARRAY_SIZE(uart_pads));
-
 	return 0;
 }
 
@@ -152,8 +144,6 @@
 
 	timer_init();
 
-	preloader_console_init();
-
 	/* Clear the BSS. */
 	memset(__bss_start, 0, __bss_end - __bss_start);
 
@@ -163,6 +153,8 @@
 		hang();
 	}
 
+	preloader_console_init();
+
 	enable_tzc380();
 
 	/* DDR initialization */