ARM: bitmain: Enable legacy u-boot format

Still legacy formats are used that's why enable them by default.
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

configs/bitmain_antminer_s9_defconfig

@@ -19,6 +19,7 @@ CONFIG_SYS_LDSCRIPT="arch/arm/mach-zynq/u-boot.lds"
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_FIT_VERBOSE=y
+CONFIG_LEGACY_IMAGE_FORMAT=y
 CONFIG_BOOTDELAY=3
 CONFIG_USE_PREBOOT=y
 # CONFIG_DISPLAY_CPUINFO is not set