arm: soc: nxp k6x: Add Initial support for NXP MPU

This patch adds initial MPU support to NXP K6x family.
The boot configuration prevents the following security issues:
* Prevent to read at an address that is reserved in the memory map.
* Prevent to write into the boot Flash/ROM.
* Prevent from running code located in SRAM.

This driver has been tested on FRDM-K64F.

Change-Id: I907168fff0c6028f1c665f1d3c224cbeec31be32
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>