TZ: Add configuration switch for PXN

PXN is a feature for armv8.1. Make it as an optional feature.
Default value is 'OFF'. Set 'TFM_PXN_ENABLE' to 'ON' when use it.

Change-Id: I3c7a15908dc37a9617894b2a0a53c7fcb1f35e2c
Signed-off-by: Summer Qin <summer.qin@arm.com>
8 files changed