Resolve "Add rpi3 overclock presets"

Please make sure your MR is ready to be merged !

• You added the changes in CHANGELOG.md
• You described the MR as below

Fixes #21 (closed)

Changes :

• added rpi3 presets
• uncommented rpi2 turbo/extrem presets
• added a warning popup if turbo/extrem preset are selected
• updated .pot file

Related to : https://gitlab.com/recalbox/recalbox-buildroot-prime/merge_requests/18