config: enable CONFIG_CMA on x86_64 (bsc#1189109).
    
    CMA was enabled in SLE15-SP3 via jsc#SLE-17227.  One difference is that
    v5.10-rc1 (b7176c261cd) upstream added the ability to allocate areas for
    each NUMA node, which changed some of the defaults.
    
    The default number of areas (19) is used here.