OvmfPkg: add and use industry standard macro PIIX4_PMBA_MASK
    
    We already have the identical purpose (but different value) macro for
    ICH9, namely ICH9_PMBASE_MASK in
    "OvmfPkg/Include/IndustryStandard/Q35MchIch9.h".
    
    Also, stop bit-negating signed integer constants.
    
    Cc: Gabriel Somlo <somlo@cmu.edu>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1333238
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
    Tested-by: Gabriel Somlo <somlo@cmu.edu>