OvmfPkg: Add bhyve support into AcpiTimerLib

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>