ANDROID: GKI: Add API to create pagetable mappings.

This is modified version of commit 5ea86bc10f628a06a
to only include the ABI diff, i.e. the addition of
the create_pgtable_mapping() function.

Test: build
Bug: 150811237
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
(cherry picked from commit 5ea86bc10f628a06ae97a837616483038d943542)
[hridya: added EXPORT_SYMBOL_GPL, documentation]
Signed-off-by: Hridya Valsaraju <hridya@google.com>
Change-Id: Ia5cd5f350d63fbe56b592f7f7190cb5e59317647