rv1108: Fix boot regression

Since commit 79030a48 ("rockchip: Add Single boot image
(with binman, pad_cat)") the following boot regression is seen:

U-Boot 2020.04-rc3-00050-gd16e18ca6c-dirty (Mar 09 2020 - 11:40:07 -0300)

Model: Elgin RV1108 R1 board
DRAM:  128 MiB
initcall sequence 67fd12a0 failed at call 6000b927 (err=-22)

This happens because the above commit missed to include the
"rockchip-u-boot.dtsi" for rv1108, so include this file
like it done for other Rockchip SoC dtsi's.

Fixes: 79030a48 ("rockchip: Add Single boot image (with binman, pad_cat)")
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>