nxp_kinetis: Add Kconfig options to select the SoC part number

The KSDK device header files require a preprocessor macro that defines
the part number string (e.g., MK64FN1M0VMD12). Create a hidden Kconfig
option to hold the part number string, and hidden Kconfig options that
the board Kconfig will use to select the specific part number.

Change-Id: I612e785026261e425b47b5b7fae0c65b4f94b30b
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>