powerpc: Cleanup vpa code

register_vpa() doesn't actually do a VPA register call it just uses the flags
you pass it, so rename it to vpa_call() to be clearer.

We can then define register_vpa() and unregister_vpa() which are both simple
wrappers around vpa_call(). (we'll need unregister_vpa() for kexec soon)

We can then cleanup vpa_init(), and because vpa_init() is only called from
platforms/pseries we remove the definition in asm-ppc64/smp.h.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>

include/asm-ppc64/smp.h

@@ -85,14 +85,6 @@ extern void smp_generic_take_timebase(void);
 
 extern struct smp_ops_t *smp_ops;
 
-#ifdef CONFIG_PPC_PSERIES
-void vpa_init(int cpu);
-#else
-static inline void vpa_init(int cpu)
-{
-}
-#endif /* CONFIG_PPC_PSERIES */
-
 #endif /* __ASSEMBLY__ */
 
 #endif /* !(_PPC64_SMP_H) */