ArmPkg/ArmLib: ASSERT on set/way cache ops being used with MMU on

ArmPkg/ArmLib: ASSERT on set/way cache ops being used with MMU on

On ARMv7 and up, doing cache maintenance by set/way is only
permitted in the context of on/offlining a core, and any other
uses should be avoided. Add ASSERT()s in the right place to
ensure that any uses with the MMU enabled are caught in DEBUG
builds.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>