arm: Add definitions for pte_mkexec/pte_mknexec

Other architectures define pte_mkexec to mark a pte as executable.
Add pte_mkexec for ARM to get the same functionality. Although no
other architectures currently define it, also add pte_mknexec to
explicitly allow a pte to be marked as non executable.

Change-Id: I673b48a1d93bf00682edbb3609a478eb28eb67a1
CRs-Fixed: 498398
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>