ARM: sunxi_defconfig: enable MODULE_UNLOAD

Loadable module support is enabled so we should allow unloading
modules too. This also allows lsmod to show values other than "-2" in
the "Used by" column.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>