imx: remove SATA boot mode for i.MX 6UL and 6ULL

The NXP i.MX 6UL and 6ULL do not support SATA and have no SATA
boot mode, hence remove it from the boot device detecion. This
fixes a build error introduced with 3bd1642d ("imx: fix USB
boot mode detection for i.MX 6UL and 6ULL")

Fixes: 3bd1642d ("imx: fix USB boot mode detection for i.MX 6UL and 6ULL")
Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Jagan Teki <jagan@openedev.com>