am335x: dfu: disable DFU in am335x_evm SPL build

Future patches will make DFU too large to fit in this board's SPL build.

Signed-off-by: Mateusz Zalega <m.zalega@samsung.com>
Acked-by: Tom Rini <trini@ti.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>