sunxi: Kconfig: Make SPL_FEL a toplevel Kconfig option

It's unfortunate that this needs to be present in both .config and spl/.config
since it makes it slightly hard to enable FEL mode for a regular defconfig. It
can be done with:
    echo CONFIG_SPL_FEL=y >> .config
    echo CONFIG_SPL_FEL=y >> spl/.config

Ideally only one of those would be needed.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>