board: toradex: verdin-imx8mm: add README

Add README with build steps for U-boot and TF-A for Verdin iMX8M Mini SoM.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>

diff --git a/board/toradex/verdin-imx8mm/README b/board/toradex/verdin-imx8mm/README
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+U-Boot for the Toradex Verdin iMX8M Mini Module
+
+Quick Start
+===========
+
+- Build the ARM trusted firmware binary
+- Get the DDR firmware
+- Build U-Boot
+- Flash to eMMC
+- Boot
+
+Get and Build the ARM Trusted Firmware (Trusted Firmware A)
+===========================================================
+
+$ echo "Downloading and building TF-A..."
+$ git clone -b imx_4.14.98_2.3.0 https://source.codeaurora.org/external/imx/imx-atf
+$ cd imx-atf
+
+Please edit `plat/imx/imx8mm/include/platform_def.h` so it contains proper
+values for UART configuration and BL31 base address (correct values listed
+below):
+#define BL31_BASE              0x910000
+#define IMX_BOOT_UART_BASE     0x30860000
+#define DEBUG_CONSOLE          1
+
+Then build ATF (TF-A):
+$ make PLAT=imx8mm bl31
+
+Get the DDR Firmware
+====================
+
+$ wget https://www.nxp.com/lgfiles/NMG/MAD/YOCTO/firmware-imx-8.4.1.bin
+$ chmod +x firmware-imx-8.4.1.bin
+$ ./firmware-imx-8.4.1.bin
+$ cp firmware-imx-8.4.1/firmware/ddr/synopsys/lpddr4*.bin ./
+
+Build U-Boot
+============
+
+$ export CROSS_COMPILE=aarch64-linux-gnu-
+$ make verdin-imx8mm_defconfig
+$ make flash.bin
+
+Flash to eMMC
+=============
+
+> tftpboot ${loadaddr} flash.bin
+> setexpr blkcnt ${filesize} + 0x1ff && setexpr blkcnt ${blkcnt} / 0x200
+> mmc dev 0 1 && mmc write ${loadaddr} 0x2 ${blkcnt}
+
+As a convenience, instead of the last two commands one may also use the update
+U-Boot wrapper:
+> run update_uboot
+
+Boot
+====
+
+ATF, U-boot proper and u-boot.dtb images are packed into FIT image,
+which is loaded and parsed by SPL.
+
+Boot sequence is:
+SPL ---> ATF (TF-A) ---> U-boot proper
+
+Output:
+U-Boot SPL 2020.01-00187-gd411d164e5 (Jan 26 2020 - 04:47:26 +0100)
+Normal Boot
+Trying to boot from MMC1
+NOTICE:  Configuring TZASC380
+NOTICE:  RDC off
+NOTICE:  BL31: v2.0(release):rel_imx_4.14.98_2.3.0-0-g09c5cc994-dirty
+NOTICE:  BL31: Built : 01:11:41, Jan 25 2020
+NOTICE:  sip svc init
+
+
+U-Boot 2020.01-00187-gd411d164e5 (Jan 26 2020 - 04:47:26 +0100)
+
+CPU:   Freescale i.MX8MMQ rev1.0 at 0 MHz
+Reset cause: POR
+DRAM:  2 GiB
+MMC:   FSL_SDHC: 0, FSL_SDHC: 1, FSL_SDHC: 2
+Loading Environment from MMC... OK
+In:    serial
+Out:   serial
+Err:   serial
+Model: Toradex Verdin iMX8M Mini Quad 2GB Wi-Fi / BT IT V1.0A, Serial# 06535149
+Net:   eth0: ethernet@30be0000
+Hit any key to stop autoboot:  0
+Verdin iMX8MM #