boards: arm: lpcxpresso55s69: Force TF-M when using NS boards

This commit forces the `CONFIG_BUILD_WITH_TFM=y` option when
using the `_ns` build targets for the LPC55sXX.

Using these targets in samples or in CI without an accompanying
secure environment image (ex. TF-M) leads to execution failures,
since the NS images are offset a predetermined amount.

Fixes #35100

Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>