arm: am57xx: env: Fix DFU variables

Commit 8502fe84 ("configs: am57xx_evm: define
CONFIG_SPL_LOAD_FIT_ADDRESS for SPL-DFU") implements incorrect ifdef
logic, which leads to DFU variables absence in non-SPL environment. Fix
that in order to bring back DFU variables, by reflecting the logic in
include/configs/dra7xx_evm.h.

Fixes: 8502fe84 ("configs: am57xx_evm: define CONFIG_SPL_LOAD_F...")
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>