MIPS: Cleanup code about plat_mem_setup()

In the current code, plat_mem_setup() is called by arch_mem_init() instead
of setup_arch() and has been declared in asm/bootinfo.h, so modify the code
comment to reflect the reality and remove the useless duplicate declartion
in arch/mips/kernel/setup.c.
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/bootinfo.h

@@ -121,7 +121,7 @@ extern unsigned long fw_passed_dtb;
 #endif
 
 /*
- * Platform memory detection hook called by setup_arch
+ * Platform memory detection hook called by arch_mem_init()
  */
 extern void plat_mem_setup(void);
 

arch/mips/kernel/setup.c

@@ -654,8 +654,6 @@ static void __init bootcmdline_init(char **cmdline_p)
  */
 static void __init arch_mem_init(char **cmdline_p)
 {
-	extern void plat_mem_setup(void);
-
 	/* call board setup routine */
 	plat_mem_setup();
 	memblock_set_bottom_up(true);