Assuming you are referring to the IA32 architecture, unfortunately, there is no 'generic' memory event that you could count.
You can do what you ask if you are specific about the memory accesses. For example, you could count '2nd-level cache read misses' and then determine what percent of all instructions retired that was. See the Event Ratios, which allow you to select predefined ratios as well as define your own.
Regards,
