doc: imx: mkimage: reorganize i.MX mkimage documentation

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>

diff --git a/doc/imx/README.imximage b/doc/imx/mkimage/README.imximage
similarity index 100%
rename from doc/imx/README.imximage
rename to doc/imx/mkimage/README.imximage

diff --git a/doc/imx/README.mxsimage b/doc/imx/mkimage/README.mxsimage
similarity index 100%
rename from doc/imx/README.mxsimage
rename to doc/imx/mkimage/README.mxsimage