Please download "Source: QorIQ Linux SDK v2.0" ISO from https://www.nxp.com/design/software/embedded-software/linux-software-and-development-tools/linux-sdk...
After install this ISO, you could get u-boot source code tar ball in downloads folder.
b25805@sun:/backup/b25805/QorIQ-SDK-V2.0-20160527-yocto$ ls downloads/ | grep u-boot
git2_git.freescale.com.ppc.sdk.u-boot.git.tar.gz
In addition, you could create a build project "source ./fsl-setup-env -m t1023rdb", run the following command to extract u-boot source code.
$ bitbake u-boot -c patch -f
You will get u-boot source code in folder build_t1023rdb/tmp/work/t1023rdb-fsl-linux/u-boot-qoriq/2016.09+fslgit-r0/git/.