x86/xen: Fix kerneldoc warning

Fix the following W=1 kernel warnings:

arch/x86/xen/setup.c:725: warning: expecting prototype for
machine_specific_memory_setup(). Prototype was for xen_memory_setup()
instead.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20220307062554.8334-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

arch/x86/xen/setup.c

@@ -719,7 +719,7 @@ static void __init xen_reserve_xen_mfnlist(void)
 }
 
 /**
- * machine_specific_memory_setup - Hook for machine specific memory setup.
+ * xen_memory_setup - Hook for machine specific memory setup.
  **/
 char * __init xen_memory_setup(void)
 {