[AMDGPU] Add _e64_dpp asm suffix to docs

The _e64_dpp suffix can be added to an instruction to force the
AsmParser to encode it as VOP3 with DPP if possible on GFX11+. This has
been the behavior since GFX11 was introduced; this patch only updates
the documentation.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D153564