powerpc: Move definitions of secondary CPU spinloop to header file

Those definitions are currently declared extern in the .c file where
they are used, move them to a header file instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/85xx/smp.c

@@ -25,7 +25,6 @@
 
 #include <sysdev/fsl_soc.h>
 
-extern volatile unsigned long __secondary_hold_acknowledge;
 extern void __early_start(void);
 
 #define BOOT_ENTRY_ADDR_UPPER	0