make_dtb.sh: add include path

The device tree include file for TI TPS65* is in a
relative path to the source for example:
device-tree/src/arm/ti/omap/am335x-bone-common.dtsi#n305
device-tree/src/arm/rockchip/rk3066a-marsboard.dts#n183

This patch gets the dts path and adds that as an include path
for the device tree compiler.

Approved by: manu (mentor)
Differential revision: https://reviews.freebsd.org/D53887