ARM64: dts: marvell: armada-3720 add RTC support

The Armada 3720 DB board has an RTC on the I2C bus. It's a PT7C4337A from
Pericom but which claims to be fully compatible with the ds1337.
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>

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

@@ -98,6 +98,12 @@ gpio_exp: pca9555@22 {
 		 * IO0_7: PWR_EN_SGMII	IO1_7: PWR_MCI_PS
 		 */
 	};
+
+	rtc@68  {
+		/* PT7C4337A from pericom fully compatible with the ds1337 */
+		compatible = "dallas,ds1337";
+		reg = <0x68>;
+	};
 };
 
 /* CON3 */