imx: imx8m[m/n]_beacon: Enable SPL_DM_SERIAL

Enable CONFIG_SPL_DM_SERIAL. uart2 and its pinmux was already
marked with u-boot,dm-spl.
Move preloader_console_init after spl_init to make sure driver
model work.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Adam Ford <aford173@gmail.com> #imx8mm_beacon
Reviewed-by: Fabio Estevam <festevam@denx.de>
Tested-by: Adam Ford <aford173@gmail.com> #imx8mn_beacon
diff --git a/board/beacon/imx8mm/spl.c b/board/beacon/imx8mm/spl.c
index 12266b2..f92b4c3 100644
--- a/board/beacon/imx8mm/spl.c
+++ b/board/beacon/imx8mm/spl.c
@@ -59,14 +59,8 @@
 }
 #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[] = {
-	IMX8MM_PAD_UART2_RXD_UART2_RX | MUX_PAD_CTRL(UART_PAD_CTRL),
-	IMX8MM_PAD_UART2_TXD_UART2_TX | MUX_PAD_CTRL(UART_PAD_CTRL),
-};
-
 static iomux_v3_cfg_t const wdog_pads[] = {
 	IMX8MM_PAD_GPIO1_IO02_WDOG1_WDOG_B  | MUX_PAD_CTRL(WDOG_PAD_CTRL),
 };
@@ -79,8 +73,6 @@
 
 	set_wdog_reset(wdog);
 
-	imx_iomux_v3_setup_multiple_pads(uart_pads, ARRAY_SIZE(uart_pads));
-
 	return 0;
 }
 
@@ -128,8 +120,6 @@
 
 	timer_init();
 
-	preloader_console_init();
-
 	/* Clear the BSS. */
 	memset(__bss_start, 0, __bss_end - __bss_start);
 
@@ -139,6 +129,8 @@
 		hang();
 	}
 
+	preloader_console_init();
+
 	ret = uclass_get_device_by_name(UCLASS_CLK,
 					"clock-controller@30380000",
 					&dev);