arm: Add XN bit to pgrprot_stronglyordered

pgprot_stronglyordered sets page attributes to the strongly ordered
memory type. This type is often used for regions of memory which
cannot be speculated into. Strongly ordered memory by itself prevents
speculative data accesses but it will not prevent speculative instruction
accesses. Since strongly ordered memory will never be used for
instructions, set the XN bit as well to prevent speculative instruction
fetches.

Change-Id: I3405a6f791499f32d94b0a60f55f2ffa08620c85
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>