trafgen: xilinx: Use proper MasterRam offset

In the latest Axi Traffic generator core the MasterRam offset
is changed from 0x10000 to 0xc000

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>