If you are using a Master page in your application then can add the script there or use a seperate JS file add the script there to refer into individual pages globally .
You can add a common class for all the textboxes which you want to do spell check
$Spelling.SpellCheckInWindow('.MyClassName')
Or you can use textinputs which enables the spell check for all the input type text
$Spelling.SpellCheckInWindow('textinputs')
Hope this helps