Unfortunately, there is no UIRequiredDeviceCapabilities
string that only filters for devices with the A5 processor.
The closest you get, looking at the "Device Compatibility Matrix" in the new iTunes Connect Developer Guide, is the new bluetooth-le
key that filters for just the iPhone 4S based on its support for Bluetooth 4.0 Low Power communication. The iPad 2 doesn't have this hardware (although it's safe to bet the next generation will), so it doesn't satisfy this required capability.
I'm a little surprised by this, because there are new OpenGL ES extensions which are only available on the iPad 2 and iPhone 4S, and I could see building applications that require those capabilities. I'm not sure how you'd specify that, so I filed an enhancement request (rdar://10356232) for a key like this.
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