add SPI RTC overlay

This currently supports PCF2123 chips on SPI0.0

arch/arm/boot/dts/overlays/Makefile

@@ -78,6 +78,7 @@ 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

@@ -974,6 +974,10 @@ Info:   move SPI function block to GPIO 35 to 39
 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

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

@@ -0,0 +1,31 @@
+/dts-v1/;
+/plugin/;
+
+/ {
+  compatible = "brcm,bcm2708";
+
+  fragment@0 {
+    target = <&spidev0>;
+    __dormant__ {
+      status = "disabled";
+    };
+  };
+
+  fragment@1 {
+    target = <&spi0>;
+    __dormant__ {
+      #address-cells = <1>;
+      #size-cells = <0>;
+      rtc-pcf2123@0 {
+        compatible = "nxp,rtc-pcf2123";
+        spi-max-frequency = <5000000>;
+        spi-cs-high = <1>;
+        reg = <0>;
+      };
+    };
+  };
+
+  __overrides__ {
+    pcf2123 = <0>, "=0=1";
+  };
+};