A couple of OpenBoot Prom variables max-pci-depth and hotplug-memsize
are pre-defined as 9 and 0x40000000 respectively for all current
platforms.  Either one is not enough for Supernova platforms which
need to support IOBox and PLX 8548/8648 reconfiguration.
The trend is to move configuration and platform specific information
from OpenBoot Prom to Machine Description.

    "max-pci-depth" 

    Optional max-pci-depth property for sun4v to PCI Express root nexus
    device MD nodes.  An integer to specify the allowed maximum level of
    pci device tree created by OpenBoot for the system.

    If this property is missing in IO device MD then the maximum depth is
    defined by the implementation of OpenBoot.

    "hotplug-memsize" 

    Optional hotplug-memsize property for sun4v to PCI Express root nexus
    device  MD nodes.  An integer to specify the allowed maximum size of
    PCI MEM32 space used by OpenBoot to do PCI Hotplug Resource Preallocation
    as defined in FWARC/2006/198.

    If this property is missing in IO device MD then the maximum PCI MEM32
    size is defined by the implementation of OpenBoot.

    "dma-sync-options" 

    Optional dma-sync-options property for sun4v to PCI Express root nexus
    device  MD nodes. An integer to specify the system syncing requirement
    for Solaris. Refers to FWARC/2009/213 for detailed information.