[NFC] Fixed -Wdocumentation warning

/srv/llvm-buildbot-srcatch/llvm-build-dir/clang-x86_64-debian-fast/llvm.src/lib/Target/AMDGPU/AMDGPUGenRegisterBankInfo.def:98:1: warning: not a Doxygen trailing comment [-Wdocumentation]
1 warning generated.

llvm-svn: 370596