Normally, CMSIS headers are retrieved as package from ST git repository
for each stm32.
However stm32mp1 family does not have a CMSIS headers repository but
have been included into RIOT source code in a previous commit.
For stm32mp1, CMSIS headers package must then not be retrieved and
vectors have to be generated from already in-source CMSIS headers.
Signed-off-by: Gilles DOFFE <gilles.doffe@savoirfairelinux.com>