Binding a keyboard shortcut to shut down
You don't need to map the power button, that menu is already mapped to control+eject



Other useful shutdown/restart/logoff key mappings are below which might be of interest too  

Prompt to save work and restart the Mac - Control+Command+Eject  

Prompt to save work and shutdown the Mac - Control+Option+Command+Eject  

Sleep - Command+Option+Eject  

Sleep displays - Shift+Control+Eject  

If the computer isn't responding and you need to force it to power off, you can hold down the power button for 6 seconds, you'll lose unsaved work though.  

These power shortcuts are listed on the apple support pages