board: nrf53: Fix build error for thingy53_nrf5340 when MPU is disabled

Fix build error for thingy53_nrf5340 board when built CONFIG_ARM_MPU=n.
In this configuration soc.h is not included from board.c, which leaves
NRF_SPU and NRF_RESET undefined.

Remove unused header soc_secure.h from nrf5340dk_nrf5340 board.
The GPIO configuration that used soc_secure.h has been moved out of this
file.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>