Update CHANGELOG for 3.18.0 and collect Tested-by tags.

Tested-by: Igor Opaniuk <igor.opaniuk@foundries.io> (Poplar)
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (vexpress-qemu_virt, GP, PKCS#11)
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (Juno)
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (FVP Base RevC)
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (FVP Foundation model)
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (imx-mx8mqevk)
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (vexpress-qemu_armv8a, GP, PKCS#11)
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (hikey-hikey960, GP, PKCS#11)
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (hikey-hikey, GP, PKCS#11)
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-mx8dxlevk)
Tested-by: Clement Faure <clement.faure@nxp.com> (imx-mx8ulpevk)
Tested-by: Etienne Carriere <etienne.carriere@linaro.org> (stm32mp1, gp, pkcs11, stmm)
Tested-by: Sahil Malhotra <sahil.malhotra@nxp.com> (LS1012A-RDB)
Tested-by: Sahil Malhotra <sahil.malhotra@nxp.com> (LS1028A-RDB)
Tested-by: Sahil Malhotra <sahil.malhotra@nxp.com> (LS1088A-RDB)
Tested-by: Sahil Malhotra <sahil.malhotra@nxp.com> (LS2088A-RDB)
Tested-by: Sahil Malhotra <sahil.malhotra@nxp.com> (LS1046A-RDB)
Tested-by: Sahil Malhotra <sahil.malhotra@nxp.com> (LS1043A-RDB)
Tested-by: Sahil Malhotra <sahil.malhotra@nxp.com> (LX2160A-RDB)
Tested-by: Sahil Malhotra <sahil.malhotra@nxp.com> (LX2160A-QDS)
Tested-by: Ricardo Salveti <ricardo@foundries.io> (ZynqMP)
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>