soc: intel_adsp: tools: improve cavstool.py winstream code

Modify the winstream code in cavstool.py to use the Regs helper
class and get rid of byte array access when reading and writing
to winstream headers. This brings the cavstool.py implementation
closer to the Zephyr C reference implementation and ensures
the header fields are read and written to with 32bit access.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>