LoongArch: Fix build error if CONFIG_SUSPEND is not set

We can see the following build error on LoongArch if CONFIG_SUSPEND is
not set:

  ld: drivers/acpi/sleep.o: in function 'acpi_pm_prepare':
  sleep.c:(.text+0x2b8): undefined reference to 'loongarch_wakeup_start'

Here is the call trace:

  acpi_pm_prepare()
    __acpi_pm_prepare()
      acpi_sleep_prepare()
        acpi_get_wakeup_address()
          loongarch_wakeup_start()

Root cause: loongarch_wakeup_start() is defined in arch/loongarch/power/
suspend_asm.S which is only built under CONFIG_SUSPEND. In order to fix
the build error, just let acpi_get_wakeup_address() return 0 if CONFIG_
SUSPEND is not set.

Fixes: 366bb35a8e ("LoongArch: Add suspend (ACPI S3) support")
Reviewed-by: WANG Xuerui <git@xen0n.name>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/all/11215033-fa3c-ecb1-2fc0-e9aeba47be9b@infradead.org/
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/acpi.h

@@ -41,8 +41,11 @@ extern void loongarch_suspend_enter(void);
 
 static inline unsigned long acpi_get_wakeup_address(void)
 {
+#ifdef CONFIG_SUSPEND
 	extern void loongarch_wakeup_start(void);
 	return (unsigned long)loongarch_wakeup_start;
+#endif
+	return 0UL;
 }
 
 #endif /* _ASM_LOONGARCH_ACPI_H */