Makefile: socfpga: Add target to generate hex output for combined spl and dtb

Add target to Makefile to generate "u-boot-spl-dtb.hex" for Intel
SOCFPGA SOC64 devices (Stratix 10 and Agilex). "u-boot-spl-dtb.hex"
is hex formatted spl with and offset of CONFIG_SPL_TEXT_BASE. It
combines the spl image and dtb. "u-boot-spl-dtb.hex" is needed to
generate the final configuration bitstream for Intel SOCFPGA SOC64
devices.

Signed-off-by: Dalon Westergreen <dalon.westergreen@intel.com>
Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>
diff --git a/Makefile b/Makefile
index 68ce519..d6eda45 100644
--- a/Makefile
+++ b/Makefile
@@ -1264,11 +1264,6 @@
 		$(if $(CONFIG_X86_16BIT_INIT),-R .start16 -R .resetvec) \
 		$(if $(CONFIG_MPC85XX_HAVE_RESET_VECTOR),-R .bootpg -R .resetvec)
 
-OBJCOPYFLAGS_u-boot-spl.hex = $(OBJCOPYFLAGS_u-boot.hex)
-
-spl/u-boot-spl.hex: spl/u-boot-spl FORCE
-	$(call if_changed,objcopy)
-
 binary_size_check: u-boot-nodtb.bin FORCE
 	@file_size=$(shell wc -c u-boot-nodtb.bin | awk '{print $$1}') ; \
 	map_size=$(shell cat u-boot.map | \
@@ -1940,6 +1935,12 @@
 	@:
 	$(SPL_SIZE_CHECK)
 
+spl/u-boot-spl-dtb.bin: spl/u-boot-spl
+	@:
+
+spl/u-boot-spl-dtb.hex: spl/u-boot-spl
+	@:
+
 spl/u-boot-spl: tools prepare \
 		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_SPL_OF_PLATDATA),dts/dt.dtb) \
 		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_TPL_OF_PLATDATA),dts/dt.dtb)