include/configs: ls1046afrwy: add support for boot targets.

ls1046afrwy does not support sata boot.
So Override default BOOT_TARGET_DEVICES
and support only  mmc, usb and dhcp as boot targets.

Signed-off-by: Pramod Kumar <pramod.kumar_1@nxp.com>
[Rebased]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>