Here is the step-by-step to boot U-Boot on rv1108 evb.

Get ddr init binary
==============================================================================
  > git clone  https://github.com/rockchip-linux/rkbin.git
  > dd if=./rkbin/rv1x/rv1108ddr.bin of=ddr.bin bs=4 skip=1

Compile  U-Boot
===========================
  > make CROSS_COMPILE=arm-linux-gnueabi- evb-rv1108_defconfig  all
  > ./tools/mkimage  -n rv1108 -T rksd -d ddr.bin spl.bin
  > cat spl.bin u-boot.bin > u-boot.img

Flash the image by rkdeveloptool
================================
rkdeveloptool can get from https://github.com/rockchip-linux/rkdeveloptool.git

Power on(or reset with RESET KEY) with MASKROM KEY preesed, and then:
  > rkdeveloptool db ./rkbin/rv1x/RV1108_usb_boot.bin
  > rkdeveloptool wl 0x40 u-boot.img
  > rkdeveloptool RD

You should be able to get U-Boot log message from boot console:

DDR Version V1.02 20170220
In
400MHz
DDR3
Bus Width=16 Col=10 Bank=8 Row=15 CS=1 Die Bus-Width=16 Size=512MB
mach:2
OUT


U-Boot 2017.05-00693-g3a5b171 (Jun 01 2017 - 17:37:53 +0800)

Model: Rockchip RV1108 Evaluation board
DRAM:  128 MiB
APLL: 600000000 DPLL:792000000 GPLL:384000000
MMC:
Using default environment

In:    serial@10210000
Out:   serial@10210000
Err:   serial@10210000
Net:   No ethernet found.
Hit any key to stop autoboot:  0
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