i.MX3: Add a basic default config for the imx31ads (reference only)

Currently there is a mx31ads BSP file in the kernel, but no default
config file.

This patch adds a basic default config for the imx31ads but it is for test
purposes only to ensure the i.MX2 patch stack doesn't break the i.MX3.

Note: For reference only. This configuration is untested as I have
no access to an mx31ads.
Signed-off-by: Juergen Beisert <j.beisert@pengutronix.de>