soc: arm: ti_lm3s6965: Select CPU DWT feature symbol

The Data Watchpoint and Trace (DWT) is an optional debug unit for the
Cortex-M family cores (except ARMv6-M; i.e. M0 and M0+) that provides
watchpoints, data tracing and system profiling capabilities.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>