imx: add i.MX8MN DDR4 board support

Support pinctrl/clk/sdhc, include ddr4 timing data.

Log:
U-Boot SPL 2019.10-rc3-00356-g497c500423-dirty (Sep 16 2019 - 10:54:58 +0800)
Normal Boot
Trying to boot from BOOTROM
image offset 0x8000, pagesize 0x200, ivt offset 0x0

U-Boot 2019.10-rc3-00356-g497c500423-dirty (Sep 16 2019 - 10:54:58 +0800)

CPU:   Freescale i.MX8MNano rev1.0 at 24 MHz
Reset cause: POR
Model: NXP i.MX8MNano DDR4 EVK board
DRAM:  2 GiB
MMC:   FSL_SDHC: 1, FSL_SDHC: 2
Loading Environment from MMC... *** Warning - bad CRC, using default environment

In:    serial
Out:   serial
Err:   serial
Net:   No ethernet found.
Hit any key to stop autoboot:  0

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/board/freescale/imx8mn_evk/Makefile b/board/freescale/imx8mn_evk/Makefile
new file mode 100644
index 0000000..9511a70
--- /dev/null
+++ b/board/freescale/imx8mn_evk/Makefile
@@ -0,0 +1,12 @@
+#
+# Copyright 2018 NXP
+#
+# SPDX-License-Identifier:      GPL-2.0+
+#
+
+obj-y += imx8mn_evk.o
+
+ifdef CONFIG_SPL_BUILD
+obj-y += spl.o
+obj-$(CONFIG_IMX8M_DDR4) += ddr4_timing.o
+endif