OvmfPkg: set ActiveHigh polarity for the SCI with a dedicated link device

OvmfPkg: set ActiveHigh polarity for the SCI with a dedicated link device

We cannot specify a pin-GSI connection for the SCI directly in the _PRT
because that implies ActiveLow polarity, clashing with both qemu and the
MADT we prepare.

With this patch the RHEL-6 guest logs the following:

    ACPI: PCI Interrupt Routing Table [\_SB_.PCI0._PRT]
    ACPI: PCI Interrupt Link [LNKS] (IRQs *9)
    ACPI: PCI Interrupt Link [LNKA] (IRQs 5 10 *11)
    ACPI: PCI Interrupt Link [LNKB] (IRQs 5 10 *11)
    ACPI: PCI Interrupt Link [LNKC] (IRQs 5 *10 11)
    ACPI: PCI Interrupt Link [LNKD] (IRQs 5 *10 11)

The patch amends svn rev 13625. Testing it in a RHEL-6 guest, the problems
described in
<http://sourceforge.net/mailarchive/message.php?msg_id=29660862> do not
reappear.

The code is derived from Paolo Bonzini's patch (originally appearing as
SeaBIOS commit f64a472a, "acpi: reintroduce LNKS"). Said original patch is
copyrighted by Red Hat (our common employer), and it has been relicensed
<http://sourceforge.net/mailarchive/message.php?msg_id=30393854> to form
the basis of this derived patch for edk2. The latter is therefore

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14111 6f19259b-4bc3-4df7-8a09-765794883524