tools/mkimage: Fix DTC run command to handle file names with space

fit_handle_file function does not quote input and output files while preparing
command to run DTC to convert .its to .itb. This results in a failure if input
or output files contain spaces in their names. Quote input and output files in
DTC command to avoid this failure.

Signed-off-by: Mirza, Taimoor <Taimoor_Mirza@mentor.com>