sparc64: fix sparse warnings in init_64.c

Fix following warnings:
init_64.c:191:10: warning: symbol 'dcpage_flushes' was not declared. Should it be static?
init_64.c:193:10: warning: symbol 'dcpage_flushes_xcall' was not declared. Should it be static?

Add extern declaration to asm/setup.h and drop local declaration in smp_64.h

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>