drivers: pinmux: stm32: add F4 RTS/CTS definitions

Adds USART RTS/CTS definitions based on STM32F469X/STM32F479X.

Co-authored-by: Benoit Leforestier <benoit.leforestier@gmail.com>
Signed-off-by: Georgij Cernysiov <g.cernysiov@elco-automation.de>

drivers/pinmux/stm32/pinmux_stm32f4.h

@@ -414,6 +414,8 @@
 #define STM32F4_PINMUX_FUNC_PG9_USART6_RX   \
 	(STM32_PINMUX_ALT_FUNC_8 | STM32_PUSHPULL_NOPULL)
 
+#define STM32F4_PINMUX_FUNC_PG8_USART6_RTS \
+	(STM32_PINMUX_ALT_FUNC_8 | STM32_OPENDRAIN_PULLUP)
 #define STM32F4_PINMUX_FUNC_PG8_ETH			    \
 	(STM32_PINMUX_ALT_FUNC_11 | STM32_PUSHPULL_NOPULL | \
 	 STM32_OSPEEDR_VERY_HIGH_SPEED)
@@ -424,9 +426,13 @@
 	(STM32_PINMUX_ALT_FUNC_11 | STM32_PUSHPULL_NOPULL | \
 	 STM32_OSPEEDR_VERY_HIGH_SPEED)
 
+#define STM32F4_PINMUX_FUNC_PG12_USART6_RTS \
+	(STM32_PINMUX_ALT_FUNC_8 | STM32_OPENDRAIN_PULLUP)
 #define STM32F4_PINMUX_FUNC_PG12_UART10_TX  \
 	(STM32_PINMUX_ALT_FUNC_11 | STM32_PUSHPULL_PULLUP)
 
+#define STM32F4_PINMUX_FUNC_PG13_USART6_CTS \
+	(STM32_PINMUX_ALT_FUNC_8 | STM32_OPENDRAIN_PULLUP)
 #define STM32F4_PINMUX_FUNC_PG13_ETH        \
 	(STM32_PINMUX_ALT_FUNC_11 | STM32_PUSHPULL_NOPULL | \
 	 STM32_OSPEEDR_VERY_HIGH_SPEED)
@@ -437,6 +443,8 @@
 	(STM32_PINMUX_ALT_FUNC_11 | STM32_PUSHPULL_NOPULL | \
 	 STM32_OSPEEDR_VERY_HIGH_SPEED)
 
+#define STM32F4_PINMUX_FUNC_PG15_USART6_CTS \
+	(STM32_PINMUX_ALT_FUNC_8 | STM32_OPENDRAIN_PULLUP)
 /* Port H */
 #define STM32F4_PINMUX_FUNC_PH2_ETH			    \
 	(STM32_PINMUX_ALT_FUNC_11 | STM32_PUSHPULL_NOPULL | \