Move cpus_ready_for_init() to AMD K8

The function is a noop for all but amd/serengeti_cheetah.

Change-Id: I09e2e710aa964c2f31e35fcea4f14856cc1e1dca
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/1184
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

src/northbridge/amd/amdk8/amdk8.h

@@ -14,4 +14,6 @@ void setup_resource_map_offset(const unsigned int *register_values, int max, uns
 void fill_mem_ctrl(int controllers, struct mem_controller *ctrl_a, const uint16_t *spd_addr);
 #endif
 
+void cpus_ready_for_init(void);
+
 #endif /* AMDK8_H */