spl: imx: only use HAB if spl fit signature is not enabled

There is no need to use HAB for FIT signature validation when
SPL_FIT_SIGNATURE is also enabled, as that will be validated via the
normal U-Boot signed FIT image flow.

This allows having SPL validated by HAB and the payloads to follow
being validated with FIT signatures only.

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>
Reviewed-by: Peng Fan <peng.fan@nxp.com>