mraa.c: return MRAA_SUCCESS when trying to call mraa_add_subplatform twice
mraa.c: return MRAA_SUCCESS when trying to call mraa_add_subplatform twice

If we already have a subplatform present with the same type, return success
when trying to add it again. We don't check that the tty is the same because
that is hidden inside the t_firmata* structure but we should do at some point

Signed-off-by: Brendan Le Foll <3e0e8c4069934e8b47d99704927a85c66c0ceb33@intel.com>

spitfire88/mraa,KurtE/mraa,Jon-ICS/mraa,intel-iot-devkit/mraa,KurtE/mraa,alext-mkrs/mraa,w4ilun/mraa,yongli3/mraa,yongli3/mraa,sergev/mraa,Propanu/mraa,g-vidal/mraa,malikabhi05/mraa,sergev/mraa,arfoll/mraa,Jon-ICS/mraa,whbruce/mraa,sergev/mraa,ncrastanaren/mraa,stefan-andritoiu/mraa-gpio-chardev,stefan-andritoiu/mraa,ProfFan/mraa,stefan-andritoiu/mraa-gpio-chardev,ncrastanaren/mraa,yongli3/mraa,spitfire88/mraa,sergev/mraa,Propanu/mraa,jontrulson/mraa,stefan-andritoiu/mraa,arfoll/mraa,w4ilun/mraa,KurtE/mraa,intel-iot-devkit/mraa,ncrastanaren/mraa,arfoll/mraa,spitfire88/mraa,stefan-andritoiu/mraa-gpio-chardev,w4ilun/mraa,jontrulson/mraa,alext-mkrs/mraa,whbruce/mraa,alext-mkrs/mraa,Jon-ICS/mraa,malikabhi05/mraa,jontrulson/mraa,arunlee77/mraa,Propanu/mraa,jontrulson/mraa,stefan-andritoiu/mraa,yongli3/mraa,g-vidal/mraa,Propanu/mraa,arunlee77/mraa,g-vidal/mraa,ncrastanaren/mraa,stefan-andritoiu/mraa,intel-iot-devkit/mraa,Propanu/mraa,stefan-andritoiu/mraa,intel-iot-devkit/mraa,g-vidal/mraa,ProfFan/mraa,stefan-andritoiu/mraa-gpio-chardev,arunlee77/mraa,spitfire88/mraa,intel-iot-devkit/mraa,yongli3/mraa,g-vidal/mraa,arfoll/mraa,ProfFan/mraa,alext-mkrs/mraa,malikabhi05/mraa,whbruce/mraa,ncrastanaren/mraa,w4ilun/mraa,malikabhi05/mraa,whbruce/mraa,sergev/mraa,KurtE/mraa,arunlee77/mraa,ProfFan/mraa,KurtE/mraa,malikabhi05/mraa,jontrulson/mraa,alext-mkrs/mraa,arfoll/mraa,stefan-andritoiu/mraa-gpio-chardev