[platform][vexpress-a9] fix uart to handle overruns
[platform][vexpress-a9] fix uart to handle overruns

-Mask off the rx interrupt when the circular buffer fills up.

nvll/lk,srodrig1/lk,hollanderic/lkstuff,nvll/lk,minglun-tsai/lk,nvll/lk,hollanderic/lkstuff,nvll/lk,ErikCorryGoogle/lk,srodrig1/lk,littlekernel/lk,skabet/lk,travisg/lk,skabet/lk,hollanderic/lkstuff,skabet/lk,travisg/lk,minglun-tsai/lk,sndnvaps/lk,littlekernel/lk,minglun-tsai/lk,skabet/lk,ErikCorryGoogle/lk,sndnvaps/lk,littlekernel/lk,skabet/lk,ErikCorryGoogle/lk,srodrig1/lk,travisg/lk,sndnvaps/lk,littlekernel/lk,ErikCorryGoogle/lk,minglun-tsai/lk,sndnvaps/lk,nvll/lk,travisg/lk,srodrig1/lk,hollanderic/lkstuff,srodrig1/lk,sndnvaps/lk