lib: time: update module enable MACRO

We'd better use correct way to check if module has enabled.
for we have 3 timer MACRO:
- CONFIG_TIMER
- CONFIG_SPL_TIMER
- CONFIG_TPL_TIMER

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

lib/time.c

@@ -56,7 +56,7 @@ ulong timer_get_boot_us(void)
 extern unsigned long __weak timer_read_counter(void);
 #endif
 
-#ifdef CONFIG_TIMER
+#if CONFIG_IS_ENABLED(TIMER)
 ulong notrace get_tbclk(void)
 {
 	if (!gd->timer) {