[AVR] Remove AsmOnly subtarget feature
[AVR] Added subtarget feature list
[AVR] Added PseudoSubtargetFeature
[AVR] Replace `generic` CPU with `avr1`
[AVR] Added generic CPU definitions
[AVR] Added missing SubtargetFeature `avr51`
[AVR] Removed redundant member from AVRSubtarget
[AVR] Added CPU list to AVR.td
[AVR] Fixed errors in AVR.td
[AVR] Cleaned up formatting of AVR.td
[AVR] Added getters to AVRSubtarget

Also removed redundant SupportsDES member from AVRSubtarget
[AVR] Made AVR.td more consistent
[AVR] HasRMW -> SupportsRMW
[AVR] Added subtarget predicates to AVRInstrInfo.td
[AVR] Made AVRInstrInfo.td aware of subtarget features
[AVR] Updated README