doc: mxc_hab: Move HAB related info to the appropriate doc

Currently the High Assurance Boot procedure is documented in two
places:

- doc/README.imx6
- doc/README.mxc_hab

It is better to consolidate all HAB related information into
README.mxc_hab file, so move the content from README.imx6 to
README.mxc_hab.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>