There is no need to have README in all i.MX documents name.
Remove README from i.MX docs name and add .txt file extension.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>

The Serial Download Protocol feature is availible in various
i.MX SoCs.

Move README.sdp document to imx/misc directory.

Signed-off-by: Breno Lima <breno.lima@nxp.com>

The current High Assurance Boot document README.mxc_hab
include details for the following features in a single file:

- HAB Secure Boot
- HAB Encrypted Boot

Split HAB documentation in a specific directory for a cleaner
documentation structure, subsequent patches will include more
content in HAB documentation.

Signed-off-by: Breno Lima <breno.lima@nxp.com>

The following documents describe device details according to the
i.MX family:

- README.imx25
- README.imx27
- README.imx5
- README.imx6
- README.mxs

Move all device common related document to doc/imx/common for a better
directory structure.

Signed-off-by: Breno Lima <breno.lima@nxp.com>

The following documents describe the image type used by the mkimage
tool to generate U-Boot images for i.MX devices.

- README.imximage
- README.mxsimage

Move all mkimage related document to doc/imx/mkimage for a better
directory structure.

Signed-off-by: Breno Lima <breno.lima@nxp.com>

Currently the Serial Download Protocol tools and procedure are
documented in two places:

- doc/imx/README.sdp
- doc/imx/README.imx6

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

Signed-off-by: Breno Lima <breno.lima@nxp.com>

Currently the U-Boot doc/ directory contains the following files
that are only relevant for i.MX devices:

- doc/README.imx25
- doc/README.imx27
- doc/README.imx5
- doc/README.imx6
- doc/README.imximage
- doc/README.mxc_hab
- doc/README.mxs
- doc/README.mxsimage
- doc/README.sdp

Move all content to a common i.MX folder for a better documentation
structure.

Signed-off-by: Breno Lima <breno.lima@nxp.com>