stm32/system_stm32: Reconfigure SysTick IRQ priority for L4 MCUs.

After calling HAL_SYSTICK_Config the SysTick IRQ priority is set to 15, the
lowest priority.  This commit reconfigures the IRQ priority to the desired
TICK_INT_PRIORITY value.

ports/stm32/system_stm32.c

@@ -598,8 +598,8 @@ void SystemClock_Config(void)
     HAL_PWREx_ControlVoltageScaling(PWR_REGULATOR_VOLTAGE_SCALE1);
 
     HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()/1000);
-
     HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);
+    HAL_NVIC_SetPriority(SysTick_IRQn, TICK_INT_PRIORITY, 0);
 #endif
 }