[AVR][NFC] Change the owner of the AVR backend

The former owner Dylan McKay (https://reviews.llvm.org/p/dylanmckay/)
has agreed to pass the responsibility to
Ben Shi (https://reviews.llvm.org/p/benshi001/). Please refer to
https://discourse.llvm.org/t/apply-for-the-ownership-of-the-avr-backend/70624

Reviewed By: MaskRay, lattner

Differential Revision: https://reviews.llvm.org/D150519