imx8m: Add README file for build guide

Add a brief build guide to introduce how to prepare the input images before
generating the final boot image.

Signed-off-by: Ye Li <[email protected]>

iMX8M/README

@@ -0,0 +1,30 @@
+Build Guide
+
+imx-mkimage is used to combine input images and generate final boot image with appropriate IVT set. 
+Before building the boot image, need to prepare the input images first:
+
+1. DDR PHY firmware images (Mandatory, used for all targets)
+   Files:		lpddr4_pmu_train_imem.bin and lpddr4_pmu_train_dmem.bin
+   Git:			ssh://[email protected]/imx/linux-firmware-imx.git
+   Directory:   	firmware/ddr/synopsys
+
+2. u-boot and SPL images (Mandatory, used for all targets)
+   Files:		u-boot.bin and u-boot-spl.bin
+   Git:			ssh://[email protected]/imx/uboot-imx.git
+   Directory:		./u-boot.bin and spl/u-boot-spl.bin
+   Build command:	make imx8mq_evk_defconfig; make
+
+3. ATF image (Mandatory, used for all targets)
+   File:		bl31.bin
+   Git:			ssh://[email protected]/imx/arm-trusted-firmware.git
+   Directory:		build/imx8mq/release
+   Build command:	make PLAT=imx8mq
+
+4. HDMI firmware image (flash_hdmi_spl_uboot and flash_hdmi_spl_uboot_tee)
+   File:		hdmi_imx8m.bin
+   Git:			ssh://[email protected]/imx/linux-firmware-imx.git
+   Directory:		firmware/hdmi/cadence
+
+5. Optee image (flash_hdmi_spl_uboot_tee and flash_spl_uboot_tee)
+   File:		tee.bin
+   Git:			ssh://[email protected]/imx/imx-optee-os.git