boot: zephyr: Only call sys_clock_disable when supported

Only call sys_clock_disable when the system clock driver support
this feature.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>
diff --git a/boot/zephyr/main.c b/boot/zephyr/main.c
index 148f06e..8b09c0f 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -190,7 +190,9 @@
                                      rsp->br_hdr->ih_hdr_size);
 #endif
 
-    sys_clock_disable();
+    if (IS_ENABLED(CONFIG_SYSTEM_TIMER_HAS_DISABLE_SUPPORT)) {
+        sys_clock_disable();
+    }
 
 #ifdef CONFIG_USB_DEVICE_STACK
     /* Disable the USB to prevent it from firing interrupts */