arch: enable MPU Gap filling by default in build without user mode

When we build without support for user mode, we do not need
a large number of MPU regions, so we should not allow having
MPU_GAP_FILLING unset. This would allow PRIV code execute from
SRAM, which is an unnecessary compromise on ARMv8-M builds
without USERSPACE support. We update the Kconfig dependencies
and add a sentence for clarification.

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