ARM: imx: Remove imx21 support

mx21 has not been converted to device tree yet and there is only
one single board that is supported (imx21ads).

To achieve the goal of converting all i.MX SoCs to
devicetree-only platforms, remove imx21 support.

If someone is interested in supporting imx21, patches to add
imx21 devicetree support will be welcome.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>