Question: how to convert dtb to dts file?
Answer: dtb to dts: Method-1: ... 
Method-2:
# fdtdump <dtb_file> > <dts_dump>
fdtdump command is part of device-tree-compiler package, install as below to get:
sudo apt-get install device-tree-compiler.
dts to dtb:
$KERNEL/scripts/dtc/dtc -I dts -O dtb -o <dtb_file> <dts_file>