[SPARC64]: Add prom_{start,stop}cpu_cpuid().

Use prom_startcpu_cpuid() on SUN4V instead of prom_startcpu().

We should really test for "SUNW,start-cpu-by-cpuid" presence
and use it if present even on SUN4U.

Signed-off-by: David S. Miller <davem@davemloft.net>