how to split csv columns?

You can do it by processing the process_hash field list separately and just copying the other two fields as shown below:

import csv

data = [{'device_name': 'fk6sdc2',
          rest of your data ...

def toCSV(res):
    with open('EnrichedEvents.csv', 'w', newline='', encoding='utf-8') as csvfile:
        fieldnames = 'md5,sha256,process_name,process_effective_reputation'.split(',')

        dict_writer = csv.DictWriter(csvfile, fieldnames=fieldnames, extrasaction='ignore')
        dict_writer.writeheader()

        for obj in res:
            md5, sha256 = obj['process_hash']  # Extract values from process_hash list.
            row = {'md5': md5, 'sha256': sha256}  # Initialize a row with them.
            row.update({field: obj[field]  # Copy the last two fields into it.
                            for field in fieldnames[-2:]})
            dict_writer.writerow(row)

toCSV(data)