ArmVirtPkg: enable AcpiTableDxe and EFI_ACPI_TABLE_PROTOCOL dynamically

In this patch, the ACPI protocol / driver chain is enabled dynamically,
when appropriate. This is being done in one larger patch, because
ArmVirt.dsc.inc, where AcpiTableDxe is built, is used by all the platform
DSCs.

No change in behavior should be observable after this patch on any
ArmVirtPkg platform.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>