[RISCV] Add instruction definition for dret

Summary:
The instruction dret is used to return from debug mode and is defined in the
RISC-V debug mode spec.

https://github.com/riscv/riscv-opcodes/blob/master/opcodes-system

Reviewers: apazos, asb, lenary, luismarques

Reviewed By: apazos

Subscribers: jfb, hiraditya, rbar, johnrusso, simoncook, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, MaskRay, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, Jim, s.egerton, sameer.abuasal, evandro, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78583