Build: Put CONFIG_TFM_ENABLE_MEMORY_PROTECT flag to platform

The flag "CONFIG_TFM_ENABLE_MEMORY_PROTECT" is a platform-scope
option, move it to platform_s target.
Remove the isolation setup in library SPM code and reuse the one
of SPM HAL, since they are doing the same things.

Change-Id: I5766ddecc525ee2eda3f2e5989b31dd5c41fde6b
Signed-off-by: Mingyang Sun <mingyang.sun@arm.com>