Select device for FPGA specific attributes since `intelfpga` namespace was deprecated in favor of just intel namespace

Previously we had FPGA specific attributes in `intelfpga` namespace, so it was pretty much obvious to which device they belong. Now when `intelfpga` was deprecated in favor of just `intel` namespace we can re-consider texts in some warnings etc, but it can be done in a separate patch.

_Originally posted by @MrSidims in https://github.com/intel/llvm/pull/2734#discussion_r518675525_