boards: arm: mps2_an521: some trivial Kconfig fixes

When TRUSTED_EXECUTION_NONSECUCRE is selected, we always
define the default board (mps2_an521). We do not need to
OR with TRUSTED_EXECUTION_SECURE, in this Kconfig
conditional.

In addition to that, we make the BOARD_MPS2_AN521 board
to strictly depend on the corresponding SOC, not on the
SOC series.

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