spi: if platform has one spidev, defaults to it.
spi: if platform has one spidev, defaults to it.

* Removes complexity on some platforms.

Signed-off-by: Thomas Ingleby <cea8b9d820e871fd22532c955ca84ff867a004a3@intel.com>

sundw2014/mraa,yangjae/mraa,KurtE/mraa,stefan-andritoiu/mraa,timrtoo/Intel,pctj101/mraa,SyrianSpock/mraa,malikabhi05/mraa,allela-roy/mraa,damcclos/mraa,anonymouse64/mraa,neuberfran/mraa,Meirtz/mraa,petreeftime/mraa,arunlee77/mraa,Kiritoalex/mraa,alext-mkrs/mraa,malikabhi05/mraa,sundw2014/mraa,malikabhi05/mraa,yongli3/mraa,petreeftime/mraa,Propanu/mraa,ncrastanaren/mraa,sundw2014/mraa,Hbrinj/mraa,mircea/mraa,stefan-andritoiu/mraa-gpio-chardev,KurtE/mraa,neuberfran/mraa,arfoll/mraa,Propanu/mraa,alext-mkrs/mraa,stefan-andritoiu/mraa,KurtE/mraa,stefan-andritoiu/mraa-gpio-chardev,yongli3/mraa,ProfFan/mraa,jontrulson/mraa,allela-roy/mraa,neuberfran/mraa,alexandru-elisei/mraa,w4ilun/mraa,pctj101/mraa,alexandru-elisei/mraa,yongli3/mraa,Propanu/mraa,andreivasiliu2211/mraa,neuberfran/mraa,allela-roy/mraa,intel-iot-devkit/mraa,timrtoo/Intel,pctj101/mraa,yoyojacky/mraa,anonymouse64/mraa,yongli3/mraa,ProfFan/mraa,w4ilun/mraa,timrtoo/Intel,damcclos/mraa,zBMNForks/mraa,neuroidss/mraa,alext-mkrs/mraa,matt-auld/mraa,seem-sky/mraa,sergev/mraa,mircea/mraa,ncrastanaren/mraa,whbruce/mraa,malikabhi05/mraa,g-vidal/mraa,alext-mkrs/mraa,ProfFan/mraa,ncrastanaren/mraa,matt-auld/mraa,yoyojacky/mraa,ncrastanaren/mraa,spitfire88/mraa,stefan-andritoiu/mraa,noahchense/mraa,andreivasiliu2211/mraa,ProfFan/mraa,nioinnovation/mraa,timrtoo/Intel,timrtoo/Intel,arfoll/mraa,intel-iot-devkit/mraa,stefan-andritoiu/mraa-gpio-chardev,intel-iot-devkit/mraa,stefan-andritoiu/mraa-gpio-chardev,stefan-andritoiu/mraa-gpio-chardev,jontrulson/mraa,tripzero/mraa,SyrianSpock/mraa,seem-sky/mraa,mircea/mraa,neuroidss/mraa,Pillar1989/mraa,Hbrinj/mraa,Propanu/mraa,spitfire88/mraa,g-vidal/mraa,Jon-ICS/mraa,sergev/mraa,noahchense/mraa,Pillar1989/mraa,tripzero/mraa,jontrulson/mraa,neuberfran/mraa,KurtE/mraa,arunlee77/mraa,smileboywtu/mraa,Pillar1989/mraa,whbruce/mraa,alext-mkrs/mraa,noahchense/mraa,w4ilun/mraa,sergev/mraa,stefan-andritoiu/mraa,tripzero/mraa,Kiritoalex/mraa,malikabhi05/mraa,whbruce/mraa,whbruce/mraa,yongli3/mraa,smileboywtu/mraa,KurtE/mraa,Hbrinj/mraa,petreeftime/mraa,spitfire88/mraa,arunlee77/mraa,nioinnovation/mraa,intel-iot-devkit/mraa,pctj101/mraa,STANAPO/mraa,arfoll/mraa,zBMNForks/mraa,anonymouse64/mraa,yangjae/mraa,Hbrinj/mraa,Jon-ICS/mraa,g-vidal/mraa,andreivasiliu2211/mraa,arfoll/mraa,ncrastanaren/mraa,intel-iot-devkit/mraa,noahchense/mraa,yangjae/mraa,matt-auld/mraa,Jon-ICS/mraa,noahchense/mraa,Kiritoalex/mraa,arfoll/mraa,alexandru-elisei/mraa,neuroidss/mraa,g-vidal/mraa,STANAPO/mraa,alex1818/mraa,alexandru-elisei/mraa,andreivasiliu2211/mraa,spitfire88/mraa,petreeftime/mraa,sundw2014/mraa,pctj101/mraa,alexandru-elisei/mraa,w4ilun/mraa,sergev/mraa,stefan-andritoiu/mraa,SyrianSpock/mraa,nioinnovation/mraa,damcclos/mraa,Meirtz/mraa,arunlee77/mraa,seem-sky/mraa,zBMNForks/mraa,jontrulson/mraa,Meirtz/mraa,smileboywtu/mraa,sundw2014/mraa,alex1818/mraa,alex1818/mraa,STANAPO/mraa,seem-sky/mraa,seem-sky/mraa,sergev/mraa,yangjae/mraa,g-vidal/mraa,jontrulson/mraa,Propanu/mraa,yoyojacky/mraa