pylibfdt: Support the sequential-write interface

It is useful to be able to create a device tree from scratch using
software. This is supported in libfdt but not currently available in the
Python bindings.

Add a new FdtSw class to handle this, with various methods corresponding
to the libfdt functions. When the tree is complete, calling AsFdt() will
return the completed device-tree object.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>