spi: Introduce new mcux shim driver

Adds a shim layer around the mcux dspi driver to adapt it to the Zephyr
spi interface. Unlike the existing k64 spi driver, this driver can be
used for other Kinetis SoCs that contain the dspi module.

Jira: ZEP-1374
Change-Id: I9417c1513565dfcc47ccda098492f60e840f4f84
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>