Fix VERSATILEPB boot after r331402

r331402 MFCed switch from custom DTS to upstream one. stable/11 version
still has main bus compatibility as "arm,amba-bus" while in Linux and HEAD
it has been changed to "simple-bus". simplebus(4) supports only the latter
so VERSATILEPB boot was broken by r331402. To fix this make manual change
to stable/11 version of versatile-ab.dts