boards: nxp: lpcxpresso55s69: fix TF-M support

Fix TF-M support for lpcxpresso55s69 target. The following issues were
resolved:

- lpcxpresso55s69 ns defconfig did not explicitly disable
  CONFIG_TRUSTED_EXECUTION_SECURE (which is set in the secure cpu0
  defconfig file)
- CONFIG_TFM_BOARD was not being set correctly for the NS target

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>