defconfig: mdm9630: Enable RTB

RTB logs specific reads and writes done by the Krait processor to a
special buffer that can be decoded later, and is used to determine what
physical addresses these reads and writes involved.

This is needed for analyzing apps crashes.

Change-Id: I194b9539c66a3c2e1ea34b9453d9f4507fa9aee9
Signed-off-by: Smita Ghosh <smitag@codeaurora.org>