stm32f4: fix gpio description in comment

On STM32F429 gpio PC6/PC7 can be allocated for USART6, as
reported in the comment.
But current code in
	drivers/serial/serial_stm32.c
uses a different gpio mapping (PG14/PG9) for USART6.

Fix the comment to match current code in the driver.
Signed-off-by: NAntonio Borneo <borneo.antonio@gmail.com>
To: u-boot@lists.denx.de
To: Kamil Lulko <rev13@wp.pl>
Cc: Tom Rini <trini@konsulko.com>

include/configs/stm32f429-discovery.h

@@ -55,7 +55,7 @@
  * 1:   TX:PA9  RX:PA10
  * 2:   TX:PD5  RX:PD6
  * 3:   TX:PC10 RX:PC11
- * 6:   TX:PC6  RX:PC7
+ * 6:   TX:PG14 RX:PG9
  */
 #define CONFIG_STM32_USART		1