configs: verdin-am62: Disable SPL FIT Overlay

Disable CONFIG_SPL_LOAD_FIT_APPLY_OVERLAY, this is not required nor used
on verdin am62, disable it to save precious binary size.

Update defconfig using savedefconfig, this adds CONFIG_OF_LIBFDT_OVERLAY
explicitly.

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>