boards: arm: stm32l562e_dk_ns: Add flash partitions

The current stm32l562_dk_ns has no flash partitions defined. This add
flash partitions following partition sizes that are compatible with
the TF-M platform defined at flash_layout.h and removes the redundant
overlays board files.

Signed-off-by: Gerson Fernando Budke <gerson.budke@ossystems.com.br>