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/arch/arm/mach-imx/imx8m/Kconfig b/arch/arm/mach-imx/imx8m/Kconfig
index b0932f1..eb4a73b 100644
--- a/arch/arm/mach-imx/imx8m/Kconfig
+++ b/arch/arm/mach-imx/imx8m/Kconfig
@@ -34,9 +34,16 @@
 	select SUPPORT_SPL
 	select IMX8M_LPDDR4
 
+config TARGET_IMX8MN_EVK
+	bool "imx8mn DDR4 EVK board"
+	select IMX8MN
+	select SUPPORT_SPL
+	select IMX8M_DDR4
+
 endchoice
 
 source "board/freescale/imx8mq_evk/Kconfig"
 source "board/freescale/imx8mm_evk/Kconfig"
+source "board/freescale/imx8mn_evk/Kconfig"
 
 endif