[SPARC64]: Fix build regressions added by dr-cpu changes.

Do not select HOTPLUG_CPU from SUN_LDOMS, that causes
HOTPLUG_CPU to be selected even on non-SMP which is
illegal.

Only build hvtramp.o when SMP, just like trampoline.o

Protect dr-cpu code in ds.c with HOTPLUG_CPU.

Likewise move ldom_startcpu_cpuid() to smp.c and protect
it and the call site with SUN_LDOMS && HOTPLUG_CPU.

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