To set kernel boot options globally, as an admin, open the 'Settings' page and on the 'General' tab scroll down to the 'Global Kernel Parameters' section:
Type in the desired (space separated) options and click 'Save'. The contents of the field will be used as-is. Do not use extra characters.
See MAAS CLI for how to do this with the CLI.
Per-node kernel boot options are set using the CLI. See MAAS CLI for instructions.
Note that per-node boot options take precedence to global ones.