docs: adjust the python venv create and clean

- Update pip requirements when the doc/requirements.txt
  is newer than the venv.
- Remove python venv at make doc-clean.
- Replace make with $(MAKE) to recursively call the make command

Signed-off-by: Yefeng Lin <yefeng.lin@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>

Change-Id: I8457d2fe51d26760dacf97c9bd106ebe5d5bacec