UPSTREAM: arm64/mm: add create_pgd_mapping() to create private page tables

For UEFI, we need to install the memory mappings used for Runtime Services
in a dedicated set of page tables. Add create_pgd_mapping(), which allows
us to allocate and install those page table entries early.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Tested-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

(cherry picked from commit 8ce837cee8f51fb0eacb32c85461ea2f0fafc9f8)
Signed-off-by: Tomasz Figa <tfiga@chromium.org>

Bug: 24475017

Change-Id: Ide9c47e9d6491c20a1a441768f3405772cbb7eb6
Signed-off-by: Kees Cook <keescook@google.com>