doc: fix duplicate link warning

To fix, insert the synchronous_calls.rst file inside drivers.rst; they
are both small enough. Plus, none was calling that link anyway.

Change-Id: Iba2027d143cb29497431cb47dfbcf0435087f1ae
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>