ArmLib defines a prototype for the ArmReadSctlr() function, but the AArch64 implementation is missing. So add it. Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: John Powell <john.powell@arm.com> Signed-off-by: Supreeth Venkatesh <supreeth.venkatesh@arm.com> [ardb: update commit log] Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>