While the letter X is not copy protected, the specific cross used on the controller, along with the other shapes: , are trademarked. Using your own art to represent the buttons (as in your example image), should be OK under fair use, specifically nominative fair use. Where the qualifications for nominative fair use are one of the following situations:
- The product or service cannot be readily identified without using the
trademark (e.g. trademark is descriptive of a person, place, or
- The user only uses as much of the mark as is
necessary for the identification (e.g. the words but not the font or
- The user does nothing to suggest sponsorship or endorsement
by the trademark holder. This applies even if the nominative use is
commercial, and the same test applies for metatags.
In this case, you're not trying to use the symbols as your own, but instead are directly referring to Sony's trademarked buttons, which likely falls under the first category.
If you're unsure, talk to a lawyer. If you're taking advice from someone on the internet, talk to a lawyer.