arm64: debug: mark a function as __init to save some memory

'debug_monitors_init()' is only called via 'postcore_initcall'.
It can be marked as __init to save a few bytes of memory.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20200531110015.598607-1-christophe.jaillet@wanadoo.frSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/debug-monitors.c

@@ -130,7 +130,7 @@ static int clear_os_lock(unsigned int cpu)
 	return 0;
 }
 
-static int debug_monitors_init(void)
+static int __init debug_monitors_init(void)
 {
 	return cpuhp_setup_state(CPUHP_AP_ARM64_DEBUG_MONITORS_STARTING,
 				 "arm64/debug_monitors:starting",