Update CHANGELOG for 3.21.0 and collect Tested-by tags.

Tested-by: Clement Faure <[email protected]> (imx-mx6dlsabresd)
Tested-by: Clement Faure <[email protected]> (imx-mx6qsabresd)
Tested-by: Clement Faure <[email protected]> (imx-mx6sllevk)
Tested-by: Clement Faure <[email protected]> (imx-mx6sxsabresd)
Tested-by: Clement Faure <[email protected]> (imx-mx6ulevk)
Tested-by: Clement Faure <[email protected]> (imx-mx6ullevk)
Tested-by: Clement Faure <[email protected]> (imx-mx6ulzevk)
Tested-by: Clement Faure <[email protected]> (imx-mx7dsabresd)
Tested-by: Clement Faure <[email protected]> (imx-mx7ulpevk)
Tested-by: Clement Faure <[email protected]> (imx-mx8mmevk)
Tested-by: Clement Faure <[email protected]> (imx-mx8mnevk)
Tested-by: Clement Faure <[email protected]> (imx-mx8mqevk)
Tested-by: Clement Faure <[email protected]> (imx-mx8mpevk)
Tested-by: Clement Faure <[email protected]> (imx-mx8qmmek)
Tested-by: Clement Faure <[email protected]> (imx-mx8qxpmek)
Tested-by: Clement Faure <[email protected]> (imx-mx8ulpevk)
Tested-by: Clement Faure <[email protected]> (imx-mx93evk)
Tested-by: Jens Wiklander <[email protected]> (Juno)
Tested-by: Jerome Forissier <[email protected]> (vexpress-qemu_virt)
Tested-by: Jerome Forissier <[email protected]> (vexpress-qemu_armv8a)
Tested-by: Jerome Forissier <[email protected]> (rockchip-rk3399) (RockPi4B)
Tested-by: Jens Wiklander <[email protected]> (imx-mx8mqevk)
Tested-by: Igor Opaniuk <[email protected]> (Poplar)
Tested-by: Jens Wiklander <[email protected]> (vexpress-fvp)
Tested-by: Etienne Carriere <[email protected]> (stm32mp1-157C_DK2)
Tested-by: Etienne Carriere <[email protected]> (stm32mp1-157C_EV1)
Tested-by: Joakim Bech <[email protected]> (RPi3B)
Tested-by: Ricardo Salveti <[email protected]> (ZynqMP)
Signed-off-by: Jerome Forissier <[email protected]>