stm32cube: u5: Add License file for the whole directory

The BSD 3-Clause LICENSE file provided in this commit matches
the LICENSE file referred to in files extracted from STM32CubeU5
package under directory stm32cube/stm32u5xx.

Additionally, fix license header in 2 files, which were likely
modified by the hal copy script.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>