Name: Trinity [Radeon HD 7640G]
All entries here have to be accepted by a maintainer, that's why it did not immediately get accepted. Radeon HD 7640G is right if you don't know the codename of the device. For the moment the 7xxxG and 7xxxD are named Trinity here, but it's not a problem that you did not know this codename.
Except for repeating the vendor name AMD, you did it right at first.