boards: arm: nrf5340dk_nrf5340: Rename NS target

This commit updates the NS board variant from
`nrf5340dk_nrf5340_cpuappns` to `nrf5340dk_nrf5340_cpuapp_ns`
to maintain consistency across zephyr.

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