ArmPkg/ArmMmuLib ARM: fix Mva to use idx instead of table base

ArmPkg/ArmMmuLib ARM: fix Mva to use idx instead of table base

Mva address calculation should use the left-shifted current
section index instead of the left-shifted table base address.

Using the table base address here has the side-effect of potentially
causing an access violation depending on the base address value.

Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Christopher Co <christopher.co@microsoft.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>