imx: imx8mn_beacon: Enable TrustZone

When the board was added, enabling tzc380 was left off by
mistake.

Signed-off-by: Adam Ford <aford173@gmail.com>

board/beacon/imx8mn/spl.c

@@ -122,6 +122,8 @@ void board_init_f(ulong dummy)
 		hang();
 	}
 
+	enable_tzc380();
+
 	/* DDR initialization */
 	spl_dram_init();