ARM: dts: stm32: Add alternate pinmux for mco2 pins

Add pinmux option for MCO2 pin. This is used on DHCOM when the
ethernet PHY 50 MHz clock is generated by the MCO2 on PG2 pin.
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Christophe Roullier <christophe.roullier@foss.st.com>
Cc: Gabriel Fernandez <gabriel.fernandez@foss.st.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: linux-clk@vger.kernel.org
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Tested-by: Johann Neuhauser <jneuhauser@dh-electronics.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

arch/arm/boot/dts/stm32mp15-pinctrl.dtsi

@@ -923,6 +923,21 @@ pins {
 		};
 	};
 
+	mco2_pins_a: mco2-0 {
+		pins {
+			pinmux = <STM32_PINMUX('G', 2, AF1)>; /* MCO2 */
+			bias-disable;
+			drive-push-pull;
+			slew-rate = <2>;
+		};
+	};
+
+	mco2_sleep_pins_a: mco2-sleep-0 {
+		pins {
+			pinmux = <STM32_PINMUX('G', 2, ANALOG)>; /* MCO2 */
+		};
+	};
+
 	m_can1_pins_a: m-can1-0 {
 		pins1 {
 			pinmux = <STM32_PINMUX('H', 13, AF9)>; /* CAN1_TX */