uboot-zynq: fix compile error for be short of dtc

While enable zynq uboot:
CONFIG_PACKAGE_uboot-zynq-zc702
CONFIG_PACKAGE_uboot-zynq-zed
CONFIG_PACKAGE_uboot-zynq-zybo

make will arise dtc error:
./scripts/dtc-version.sh: line 17: dtc: command not found
./scripts/dtc-version.sh: line 18: dtc: command not found
*** Your dtc is too old, please upgrade to dtc 1.4 or newer
make[4]: *** [checkdtc] Error 1

Pass the kernel dtc to uboot for compile.

Signed-off-by: Yutang Jiang <yutang.jiang@nxp.com>