arch: Add imx6sx m4 soc support

The i.MX 6SoloX SoC is a hybrid multi-core processor composed by one
Cortex A9 core and one Cortex M4 core.

Zephyr was ported to run on the M4 core. In a later release, it will
also communicate with the A9 core (running Linux) via RPMsg.

The low level drivers come from NXP FreeRTOS BSP and are located at
ext/hal/nxp/imx. More details can be found at ext/hal/nxp/imx/README

The A9 core is responsible to load the M4 binary application into the
RAM, put the M4 in reset, set the M4 Program Counter and Stack Pointer,
and get the M4 out of reset.
The A9 can perform these steps at bootloader level after the Linux
system has booted.

Signed-off-by: Stanislav Poboril <stanislav.poboril@nxp.com>