imx8mn: Add target for iMX8MN eMMC partition boot support

On iMX8MN, when booting from eMMC boot partition 0 or 1, the image
offset must be 0. It is same as eMMC fastboot case.
Add the target flash_ddr4_evk_emmc_fastboot for this DDR4 EVK.

Signed-off-by: Ye Li <ye.li@nxp.com>
diff --git a/iMX8M/soc.mak b/iMX8M/soc.mak
index a6ba3bd..990d063 100644
--- a/iMX8M/soc.mak
+++ b/iMX8M/soc.mak
@@ -179,6 +179,9 @@
 	./mkimage_imx8 -version $(VERSION) -dev flexspi -fit -loader u-boot-spl-ddr4.bin $(SPL_FSPI_LOAD_ADDR) -second_loader u-boot-ddr4-evk.itb 0x40200000 0x60000 -out $(OUTIMG)
 	./$(QSPI_PACKER) $(QSPI_HEADER)
 
+flash_ddr4_evk_emmc_fastboot: $(MKIMG) u-boot-spl-ddr4.bin u-boot-ddr4-evk.itb
+	./mkimage_imx8 -version $(VERSION) -dev emmc_fastboot -fit -loader u-boot-spl-ddr4.bin $(SPL_LOAD_ADDR) -second_loader u-boot-ddr4-evk.itb 0x40200000 0x60000 -out $(OUTIMG)
+
 flash_hdmi_spl_uboot: flash_evk
 
 flash_dp_spl_uboot: flash_dp_evk