I've done this and it works. Pasted instructions below...
First, create a new file:
<!--?xml version="1.0" encoding="UTF-8"?-->
Then update permissions and add it to launchctl:
sudo chown root:wheel /Library/LaunchDaemons/com.mysql.mysql.plist
sudo chmod 644 /Library/LaunchDaemons/com.mysql.mysql.plist
sudo launchctl load -w /Library/LaunchDaemons/com.mysql.mysql.plist
The reason you need this is because the StartupItems have been removed from Yosemite. There is a note at the start of the page in Apple's docs explaining this: Startup items are a deprecated technology. Launching of daemons through this process may be removed or eliminated in a future release of OS X.