add flag_values and flag_meanings to enum variables in CLD suite

In 3.1, the L2 CLD suite includes several (at least 5, maybe more?) variables that are enumerations which are missing the conventional attributes.

For example, the cld_phase_21 variable has attribute long_name set to "Index of the cloud phase derived for the 2100 nm products: 0 - no cloud mask, 1 - clear, 2 - water cloud, 3 - ice cloud, 4 - unknown, assume water cloud" and no flag_values or flag_meanings attributes. The correct attributes would be:

long_name: Index of the cloud phase derived for the 2100 nm products
flag_values: 0, 1, 2, 3, 4
flag_meanings: no_cloud_mask clear water_cloud ice_cloud unknown_assume_water_cloud

Remember that the flag_values data type must match the variable's data type.

cc @chamara.rajapakshe