doc: imx: reorganize i.MX documentation

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>

diff --git a/doc/README.imx25 b/doc/imx/README.imx25
similarity index 100%
rename from doc/README.imx25
rename to doc/imx/README.imx25

diff --git a/doc/README.imx27 b/doc/imx/README.imx27
similarity index 100%
rename from doc/README.imx27
rename to doc/imx/README.imx27

diff --git a/doc/README.imx5 b/doc/imx/README.imx5
similarity index 100%
rename from doc/README.imx5
rename to doc/imx/README.imx5

diff --git a/doc/README.imx6 b/doc/imx/README.imx6
similarity index 100%
rename from doc/README.imx6
rename to doc/imx/README.imx6

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

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

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

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

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