soc: arm: stm32g071: remove kernel header inclusions in soc.h

Remove the inclusion of kernel_includes.h from soc.h and replace
it with including the board-specific auto-generated headers. This
aligns the soc.h header with the current policy not to include
kernel headers in soc.h.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>