arm64: dts: marvell: armada-3720-espressobin: fill UART nodes

Fill ESPRESSObin uart0 node with pinctrl information like in the
Armada-3720-DB device tree (which uses the same node).

Also explain how to enable the second UART port available on the
headers. This second port is not enabled by default because both
headers are dedicated to expose general purpose pins and remapping
some of them to use the second UART would break existing users.
Suggested-by: NLászló ÁSHIN <laszlo@ashin.hu>
Signed-off-by: NMiquel Raynal <miquel.raynal@free-electrons.com>
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>

arch/arm64/boot/dts/marvell/armada-3720-espressobin.dts

@@ -98,9 +98,21 @@
 
 /* Exported on the micro USB connector J5 through an FTDI */
 &uart0 {
+	pinctrl-names = "default";
+	pinctrl-0 = <&uart1_pins>;
 	status = "okay";
 };
 
+/*
+ * Connector J17 and J18 expose a number of different features. Some pins are
+ * multiplexed. This is the case for instance for the following features:
+ * - UART1 (pin 24 = RX, pin 26 = TX). See armada-3720-db.dts for an example of
+ *   how to enable it. Beware that the signals are 1.8V TTL.
+ * - I2C
+ * - SPI
+ * - MMC
+ */
+
 /* J7 */
 &usb3 {
 	status = "okay";