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: Antonio Borneo <borneo.antonio@gmail.com>
To: u-boot@lists.denx.de
To: Kamil Lulko <rev13@wp.pl>
Cc: Tom Rini <trini@konsulko.com>