Update CHANGELOG for 3.21.0 and collect Tested-by tags.

Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx6dlsabresd)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx6qsabresd)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx6sllevk)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx6sxsabresd)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx6ulevk)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx6ullevk)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx6ulzevk)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx7dsabresd)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx7ulpevk)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx8mmevk)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx8mnevk)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx8mqevk)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx8mpevk)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx8qmmek)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx8qxpmek)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx8ulpevk)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx93evk)
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (Juno)
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (vexpress-qemu_virt)
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (vexpress-qemu_armv8a)
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (rockchip-rk3399) (RockPi4B)
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (imx-mx8mqevk)
Tested-by: Igor Opaniuk <igor.opaniuk@foundries.io> (Poplar)
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (vexpress-fvp)
Tested-by: Etienne Carriere <etienne.carriere@linaro.org> (stm32mp1-157C_DK2)
Tested-by: Etienne Carriere <etienne.carriere@linaro.org> (stm32mp1-157C_EV1)
Tested-by: Joakim Bech <joakim.bech@linaro.org> (RPi3B)
Tested-by: Ricardo Salveti <ricardo@foundries.io> (ZynqMP)
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>