Update CHANGELOG for 4.2.0 and collect Tested-by tags.

Signed-off-by: Joakim Bech <[email protected]>
Tested-by: Etienne Carriere <[email protected]> (stm32mp1-135F_DK w/ pkcs11)
Tested-by: Etienne Carriere <[email protected]> (stm32mp1-157C_DK2_SCMI w/ pkcs11)
Tested-by: Etienne Carriere <[email protected]> (stm32mp1-157C_EV1 w/ RPMB)
Tested-by: Imre Kis <[email protected]> (fvp-ts)
Tested-by: Jens Wiklander <[email protected]> (FVP)
Tested-by: Jens Wiklander <[email protected]> (Hikey)
Tested-by: Jens Wiklander <[email protected]> (imx-mx8mqevk)
Tested-by: Jerome Forissier <[email protected]> (rockchip-rk3399) (Rockpi4B)
Tested-by: Jerome Forissier <[email protected]> (vexpress-qemu_armv8a)
Tested-by: Jerome Forissier <[email protected]> (vexpress-qemu_virt)
Tested-by: Joakim Bech <[email protected]> (RPi 3B v1.2)
Tested-by: Sahil Malhotra <[email protected]> (imx-mx6dlsabresd...