OvmfPkg: Add bhyve support into AcpiTimerLib

On bhyve, the ACPI timer is located at a fixed IO address; it need
not be programmed into, nor fetched from, the PMBA -- power
management base address -- register of the PCI host bridge.

Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Message-Id: <20200430011212.612386-1-rebecca@bsdio.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

OvmfPkg/Library/AcpiTimerLib/BaseAcpiTimerLibBhyve.c

@@ -0,0 +1,32 @@
+/** @file
+  Provide InternalAcpiGetTimerTick for the bhyve instance of the
+  Base ACPI Timer Library
+
+  Copyright (C) 2020, Rebecca Cran <rebecca@bsdio.com>
+  Copyright (C) 2014, Gabriel L. Somlo <somlo@cmu.edu>
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+
+#include <Library/IoLib.h>
+#include <OvmfPlatforms.h>
+
+/**
+  Internal function to read the current tick counter of ACPI.
+
+  Read the current ACPI tick counter using the counter address cached
+  by this instance's constructor.
+
+  @return The tick counter read.
+
+**/
+UINT32
+InternalAcpiGetTimerTick (
+  VOID
+  )
+{
+  //
+  // Return the current ACPI timer value.
+  //
+  return IoRead32 (BHYVE_ACPI_TIMER_IO_ADDR);
+}