AMD made history when it released the “Seattle” processor, since it was its first 64-bit ARM-based processor made for servers and one of the first 64-bit ARM chips, period. It also happened to be a much stronger chip than what the ARM architecture is known for.
ARM processors are the ones that rule the mobile phone and tablet markets, thanks to their high energy efficiency compared to Intel's and AMD's x86 architecture.
Over the past half a decade, however, the ARM and x86 architectures have tried to carve out a piece of the other's turf, with limited success.
ARM actually did better, and AMD's endorsement has a lot to do with that, because it brought the ARM architecture into the server market. Micro-server market for now, but like any chip architecture, the ARM technology will scale with time.