The run script 'run/util_mmio.run' runs a test over basic
functionalities of 'Mmio::Register' and 'Mmio::Register::Subreg'. The
test covers the functions 'read' and 'bits', 'set', 'clear' and 'get'.
Inline function in 'Mmio::Register::Subreg' whose definition otherwise
looks ugly.
