target/ppc/spapr_caps: Add support for tristate spapr_capabilities
    
    spapr_caps are used to represent the level of support for various
    capabilities related to the spapr machine type. Currently there is
    only support for boolean capabilities.
    
    Add support for tristate capabilities by implementing their get/set
    functions. These capabilities can have the values 0, 1 or 2
    corresponding to broken, workaround and fixed.
    
    Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
    Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
    (cherry picked from commit 6898aed77f4636c3e77af9c12631f583f22cb5db)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>