board: ti: k3: Update the sysfw image gen repository

Now k3-image-gen[0] is the official repository for generating sysfw.itb
Update the same in AM65x and J721e README.

[0] https://git.ti.com/cgit/k3-image-gen/k3-image-gen/



Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>