If the dopesheet has focus (click it so the view's tab is underlined) then
ctrl+A will select all visible keys. So you can select the bone in the viewport, the dopesheet shows only that bone (assuming the dopesheet selection is unlocked), then
ctrl+A to select all keys for that bone.
If you select a single key in the dopesheet, then
ctrl+A will select all keys for that timeline. Pressing it again will select all visible keys.