OvmfPkg/IndustryStandard: define macros for QEMU's CPU hotplug registers

In v1.5.0, QEMU's "pc" (i440fx) board gained a "CPU present bitmap"
register block. In v2.0.0, this was extended to the "q35" board.

In v2.7.0, a new (read/write) register interface was laid over the "CPU
present bitmap", with an option for the guest to switch the register block
to the new (a.k.a. modern) interface.

Both interfaces are documented in "docs/specs/acpi_cpu_hotplug.txt" in the
QEMU tree.

Add macros for a minimal subset of the modern interface, just so we can
count the possible CPUs (as opposed to boot CPUs) in a later patch in this
series.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1515
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20191022221554.14963-3-lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

OvmfPkg/Include/IndustryStandard/I440FxPiix4.h

@@ -46,4 +46,9 @@
 #define PIIX4_PMREGMISC        0x80
 #define PIIX4_PMREGMISC_PMIOSE   BIT0
 
+//
+// IO ports
+//
+#define PIIX4_CPU_HOTPLUG_BASE 0xAF00
+
 #endif