For loading u-boot using JTAG, follow the below steps--
1. Download DDR stress test tool v.2.52 from NXP website.
2. Boot your board into serial mode, by turning all boot switches off
3. Now connect USB cable from USB OTG port to desktop/Laptop on which you will be running DDR stress test tool.
4. For information on using DDR stress test tool refer to documentation.
5. Once board is detected in DDR stress test tool, use appropriate inc script in the tool and click on download. After successful downloading of script , DDR will be initialised.
6. Now connect JTAG cable , and load u-boot.bin image using load_image command from JTAG to appropriate address, for imx6q sabresd board it is 0x17800000.
7. Once image is downloaded successfully, you can resume from same address using resume command like
For custom boards, you can use xls provided by NXP to generate inc file for custom board which can be used for loading into DDR stress test tool to initalise DDR.
I am using Arm olimex JTAG debugger for debugging.