Update CHANGELOG for 3.8.0

Update CHANGELOG for 3.8.0 and collect Tested-by tags.

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (RPi3b)
Tested-by: Igor Opaniuk <igor.opaniuk@gmail.com> (Poplar)
Tested-by: Andrew F. Davis <afd@ti.com> (plat-ti)
Tested-by: Jerome Forissier <jerome@forissier.org> (HiKey)
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (Juno)
Tested-by: Jerome Forissier <jerome@forissier.org> (HiKey960, GP)
Tested-by: Sumit Garg <sumit.garg@linaro.org> (Developerbox)
Tested-by: Jerome Forissier <jerome@forissier.org> (QEMUv8)
Tested-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org> WaRP7
Tested-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com> (Rcar H3)