uboot-imx

sunxi: Add the default mtdids and mtdparts to our env

In order for the user to be able to see and modify them, add those
variables to the default environment.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>