arm64/mm: add speculative page fault

This patch enables the speculative page fault on the arm64
architecture.

I completed spf porting in 4.9. From the test result,
we can see app launching time improved by about 10% in average.
For the apps which have more than 50 threads, 15% or even more
improvement can be got.

Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Change-Id: Ib7c8b2e354800b5023e6c6400448a6d40aaf89c8
Patch-mainline: linux-mm @ Wed, 2 May 2018 15:54:32
[vinmenon@codeaurora.org: remove the speculative fault perf counter]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
[charante@codeaurora.org: trivial merge conflict fixes]
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>