- Introduce an uart_cpu_identify() which is implemented in uart_cpu_<arch>.c

- Introduce an uart_cpu_identify() which is implemented in uart_cpu_<arch>.c
  and that can be used as an identify function for all kinds of busses on a
  certain platform. Expect for sparc64 these are only stubs right now. [1]
- For sparc64, add code to its uart_cpu_identify() for registering the on-
  board ISA UARTs and their resources based on information obtained from
  Open Firmware.
  It would be better if this would be done in the OFW ISA code. However, due
  to the common FreeBSD ISA code and PNP-IDs not always being present in the
  properties of the ISA nodes there seems to be no good way to implement that.
  Therefore special casing UARTs as the sole really relevant ISA devices on
  sparc64 seemed reasonable. [2]

Approved by:	marcel
Discussed with:	marcel [1], tmm [2]
Tested by:	make universe