ArmPkg/ArmV7Lib: add support for reading the ID_MMFR0 system register

ArmPkg/ArmV7Lib: add support for reading the ID_MMFR0 system register

Implement an accessor function for the ID_MMFR0 system register, which
contains information about the VMSA implementation. We will need this
to access the number of shareability levels and the nature of their
implementations.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18894 6f19259b-4bc3-4df7-8a09-765794883524