coreboot/haswell: make IOMMU a runtime option
    
    It is now possible to disable the IOMMU on Haswell
    boards, by doing this on your ROM image:
    
    ./nvramtool -C libreboot.rom -w iommu=Disable
    
    To enable it again, do this:
    
    ./nvramtool -C libreboot.rom -w iommu=Enable
    
    If not specified, the default behaviour is *on*.
    A follow-up patch will turn IOMMU *off* by default,
    on Dell OptiPlex 9020 SFF/MT, by setting it as such
    in cmos.default. This is to make graphics cards work
    properly to work around a bug when it's turned on.
    
    Leaving the IOMMU enabled is recommended, if it works.
    It works in most cases, including on 9020 SFF/MT when
    using the Intel GPU without a graphics card inserted.
    
    Signed-off-by: Leah Rowe <leah@libreboot.org>