sunxi: imply CONFIG_OF_LIBFDT_OVERLAY

fdt overlay support is useful for all sunxi boards, enable per default
and remove it from sunxi defconfigs.

Signed-off-by: Andre Heider <a.heider@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>