Replace license text with corresponding SPDX identifiers and update the
format of existing SPDX identifiers to follow the new guideline
Documentation/process/license-rules.rst.

Note that some of the files mentioned X11 license previously but the
license text actually matches MIT license.
Signed-off-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>