dtoc: Update fdt_util for Python 3

dtoc: Update fdt_util for Python 3

Since we are now using the bytes type in Python 3, the conversion in
fdt32_to_cpu() is not necessary, so drop it.

Also use 'int' instead of 'long' to convert the integer value, since
'long' is not present in Python 3.

With this, test_fdt passes with both Python 2 and 3:

PYTHONPATH=/tmp/b/sandbox_spl/scripts/dtc/pylibfdt python \
	./tools/dtoc/test_fdt -t

PYTHONPATH=~/cosarm/dtc/pylibfdt:tools/patman python3 \
	./tools/dtoc/test_fdt -t

Signed-off-by: Simon Glass <sjg@chromium.org>