csky: Remove compile warning for CONFIG_SMP

When CONFIG_SMP is enabled, there is a compile warning:

arch/csky/kernel/smp.c:242:6: warning: no previous prototype for
'csky_start_secondary' [-Wmissing-prototypes]
  242 | void csky_start_secondary(void)
      |      ^~~~~~~~~~~~~~~~~~~~

Add a similar prototype with csky_start in sections.h.

Signed-off-by: Guo Ren (Alibaba DAMO Academy) <guoren@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>

Author: guoren83

arch/csky/include/asm/sections.h

@@ -8,5 +8,6 @@
 extern char _start[];
 
 asmlinkage void csky_start(unsigned int unused, void *dtb_start);
+asmlinkage void csky_start_secondary(void);
 
 #endif /* __ASM_SECTIONS_H */