Update systick reload value

The systick timer is driven by the AHB bus divided by 4, set the correct
reload value to generate a timer irq every ms.

hw/bsp/gd32vf103_longan_nano/gd32vf103_longan_nano.c

@@ -39,7 +39,9 @@ void USBFS_IRQHandler(void) { tud_int_handler(0); }
 
 #define USB_NO_VBUS_PIN
 
-#define TIMER_TICKS         (SystemCoreClock / 1000)
+// According to GD32VF103 user manual clock tree:
+// Systick clock = AHB clock / 4.
+#define TIMER_TICKS         ((SystemCoreClock / 4) / 1000) 
 
 #define BUTTON_PORT         GPIOA
 #define BUTTON_PIN          GPIO_PIN_0