doc: imx: Reorganize i.MX SoC common documentation

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>
diff --git a/doc/imx/README.imx25 b/doc/imx/common/README.imx25
similarity index 100%
rename from doc/imx/README.imx25
rename to doc/imx/common/README.imx25
diff --git a/doc/imx/README.imx27 b/doc/imx/common/README.imx27
similarity index 100%
rename from doc/imx/README.imx27
rename to doc/imx/common/README.imx27
diff --git a/doc/imx/README.imx5 b/doc/imx/common/README.imx5
similarity index 100%
rename from doc/imx/README.imx5
rename to doc/imx/common/README.imx5
diff --git a/doc/imx/README.imx6 b/doc/imx/common/README.imx6
similarity index 100%
rename from doc/imx/README.imx6
rename to doc/imx/common/README.imx6
diff --git a/doc/imx/README.mxs b/doc/imx/common/README.mxs
similarity index 100%
rename from doc/imx/README.mxs
rename to doc/imx/common/README.mxs