spi: lpspi: Switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
(cherry picked from commit 6126fd836586609d6292bb5bc50b2c0a3a7eec89)
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>