xlat lib: Introduce MT_EXECUTE/MT_EXECUTE_NEVER attributes

xlat lib: Introduce MT_EXECUTE/MT_EXECUTE_NEVER attributes

This patch introduces the MT_EXECUTE/MT_EXECUTE_NEVER memory mapping
attributes in the translation table library to specify the
access permissions for instruction execution of a memory region.
These new attributes should be used only for normal, read-only
memory regions. For other types of memory, the translation table
library still enforces the following rules, regardless of the
MT_EXECUTE/MT_EXECUTE_NEVER attribute:

 - Device memory is always marked as execute-never.
 - Read-write normal memory is always marked as execute-never.

Change-Id: I8bd27800a8c1d8ac1559910caf4a4840cf25b8b0