BCM270X_DT: Add spi-rtc overlay on SPI0.0

Initial version only supports PCF2123 RTC.

See: #1510

arch/arm/boot/dts/overlays/Makefile

@@ -77,6 +77,7 @@ dtbo-$(RPI_DT_OVERLAYS) += smi.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += smi-dev.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += smi-nand.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += spi-gpio35-39.dtbo
+dtbo-$(RPI_DT_OVERLAYS) += spi-rtc.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += spi1-1cs.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += spi1-2cs.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += spi1-3cs.dtbo

arch/arm/boot/dts/overlays/README

@@ -953,6 +953,12 @@ Load:   dtoverlay=spi-gpio35-39
 Params: <None>
 
 
+Name:   spi-rtc
+Info:   Adds support for a number of SPI Real Time Clock devices
+Load:   dtoverlay=spi-rtc,<param>=<val>
+Params: pcf2123                 Select the PCF2123 device
+
+
 Name:   spi1-1cs
 Info:   Enables spi1 with a single chip select (CS) line and associated spidev
         dev node. The gpio pin number for the CS line and spidev device node

arch/arm/boot/dts/overlays/spi-rtc-overlay.dts

@@ -0,0 +1,33 @@
+/dts-v1/;
+/plugin/;
+
+/ {
+	compatible = "brcm,bcm2708";
+
+	fragment@0 {
+		target = <&spidev0>;
+		__dormant__ {
+			status = "disabled";
+		};
+	};
+
+	fragment@1 {
+		target = <&spi0>;
+		__dormant__ {
+			#address-cells = <1>;
+			#size-cells = <0>;
+			status = "okay";
+
+			rtc-pcf2123@0 {
+				compatible = "nxp,rtc-pcf2123";
+				spi-max-frequency = <5000000>;
+				spi-cs-high = <1>;
+				reg = <0>;
+			};
+		};
+	};
+
+	__overrides__ {
+		pcf2123 = <0>, "=0=1";
+	};
+};