defconfig: Enable QCOM_RTB config on SDXPRAIRIE target

Enable RTB (Register traceback) logging for compiling RTB driver and
helpful for debugging from crash logs on SDXPRAIRIE target.

Change-Id: I7a90e5e6ff2cc1e891f0f62d60dbe413e5516185
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>