StandaloneMmPkg: Fix ECC error 8005 in StandaloneMmCoreEntryPoint
    
    Bugzilla: 3150 (https://bugzilla.tianocore.org/show_bug.cgi?id=3150)
    
    The ECC tool reports error [8005] Variable name does not follow the rules:
      1. First character should be upper case
      2. Must contain lower case characters
      3. No white space characters
      4. Global variable name must start with a 'g'
    for the constants SPM_MAJOR_VER, SPM_MINOR_VER & BOOT_PAYLOAD_VERSION.
    
    Fix this by changing converting these constant variables to #defined
    values.
    
    Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
    Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
    Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
    Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>