Overwrite pl-conf.dtsi

The one included by techlab-bsp matches their FPGA BSP.
I'm using MSK BSP

recipes-bsp/device-tree/device-tree.bbappend

+FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
+
+SRC_URI_append = " \
+    file://pl-conf.dtsi \
+"
+