boards: lpcxpresso55s69: fix TFM offset for builds without MCUboot

The flash layout definition has changed in upstream TF-M for the
LCPXPRESSO55S69 platform, for builds without bootloader. Fix the
layout in the boards' configuration, as well.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>