boards: arm: lpcxpresso55s16: rename board definition

Rename the NXP LPCXpresso55S16 board definition from
lpcxpresso55s16_ns (non-secure) to lpcxpresso55s16 and remove TF-M
configuration options.

While the LPC55S16 does have Arm TrustZone support, there is no TF-M
support available upstream yet.

Fixes #35100

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>