docs: document python version used for compilation

The drm/msm driver had adopted using Python3 script to generate register
header files instead of shipping pre-generated header files. Document
the minimal Python version supported by the script.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240509-python-version-v1-1-a7dda3a95b5f@linaro.org