msm: Kconfig: enable ARCH_WANT_KMAP_ATOMIC_FLUSH for 8916

Targets with content protection (potentially) backed by
highmem need to ensure that all potential mappings are removed.
Enable ARCH_WANT_KMAP_ATOMIC_FLUSH to ensure that old kmap_atomic
mappings get flushed.

Change-Id: I9f4d7c8e59f698041671edb3e7df32164290a0a1
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>