spi: spi-fsl-dspi: Adding shutdown hook

[ Upstream commit dc234825997ec6ff05980ca9e2204f4ac3f8d695 ]

We need to ensure dspi controller could be stopped in order for kexec
to start the next kernel.
So add the shutdown operation support.

Signed-off-by: Peng Ma <peng.ma@nxp.com>
Link: https://lore.kernel.org/r/20200424061216.27445-1-peng.ma@nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>