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>