distinguesh between OneAPI SYCL CPU/GPU and OneAPI SYCL FPGA backend

[SimeonEhrig]

During the work this alpaka PR, I realized that it is required to distinguish different target processor types, if the SYCL backend is used. At the moment, the CPU and GPU device can be group together. The second target processor category is FPGA.

I should use at lease the term SYCL_DEVICE_CPU_GPU, that I can split up it later if required.