Correct write_propval_bytes() for platforms with signed char by default

Some platforms (including the Microsoft C compiler) have char defaulting
to signed.  write_propval_bytes() in the -O dts code will not behave
correctly in this case, due to sign extension.

Signed-off-by: Andrei Errapart <andrei@errapartengineering.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>