ARM: dts: meson8b: odroid-c1: prepare support for the RTC

The Odroid-C1 has the 32.768 kHz oscillator (X3 in the schematics) which
is required for the RTC. A battery can be connected separately (to the
BT1 header) - then the "rtc" node can be enabled manually. By default
the RTC is disabled because the boards typically come without the RTC
battery.
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>

arch/arm/boot/dts/meson8b-odroidc1.dts

@@ -124,6 +124,14 @@ iio-hwmon {
 		io-channels = <&saradc 8>;
 	};
 
+	rtc32k_xtal: rtc32k-xtal-clk {
+		/* X3 in the schematics */
+		compatible = "fixed-clock";
+		clock-frequency = <32768>;
+		clock-output-names = "RTC32K";
+		#clock-cells = <0>;
+	};
+
 	vcc_1v8: regulator-vcc-1v8 {
 		/*
 		 * RICHTEK RT9179 configured for a fixed output voltage of
@@ -345,6 +353,12 @@ &pwm_cd {
 	clock-names = "clkin0";
 };
 
+&rtc {
+	/* needs to be enabled manually when a battery is connected */
+	clocks = <&rtc32k_xtal>;
+	vdd-supply = <&vdd_rtc>;
+};
+
 &uart_AO {
 	status = "okay";
 	pinctrl-0 = <&uart_ao_a_pins>;