Add a new formula field and use the following calculation:
Decimal Degrees = degrees + minutes/60 + seconds/3600

Example. 120 30' 45" in decimal is 120 + (30/60) + (45/3600) = 120.5125.

Remember the " - " sign. If a latitude coordinate is in the southern hemisphere or a longitude coordinate is in the western hemisphere, be sure to put a "-" sign in front of it.

Latitudes are between 90 and -90 degrees.
Longitudes are between 180 and -180 degrees.

It is possible to do this all using text manipulation functions in a single formula using LEFT, SEARCH, etc.
See http://www.visokio.com/functions-guide
This would be quite complex, though.