[STM32F09x] Fix Serial using USART[3-8]

__HAL_GET_PENDING_IT() read IT line status in
SYSCFG register which required to be clocked.

This set of registers is implemented only on STM32F09x
to collect all pending interrupt sources associated with
each interrupt line into a single register.
This allows users to check by single read which peripheral
requires service in case more than one source is associated
to the interrupt line.

Fix #260

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/stm32/uart.c

@@ -253,6 +253,12 @@ void uart_init(serial_t *obj)
   }
 #endif
 
+#if defined(STM32F091xC) || defined (STM32F098xx)
+  /* Enable SYSCFG Clock */
+  /* Required to get SYSCFG interrupt status register */
+  __HAL_RCC_SYSCFG_CLK_ENABLE();
+#endif
+
   /* Configure GPIOs */
   /* RX */
   port = set_GPIO_Port_Clock(STM_PORT(obj->pin_rx));