arm64: dts: qcom: sa8540p-ride: enable rtc

SA8540P-ride is one of the Qualcomm platforms that does not have access
to UEFI runtime services and on which the RTC registers are read-only,
as described in:
https://lore.kernel.org/all/20230202155448.6715-1-johan+linaro@kernel.org/

Reserve four bytes in one of the PMIC registers to hold the RTC offset
the same way as it was done for sc8280xp-crd which has similar
limitations:
    commit e67b4558 ("arm64: dts: qcom: sc8280xp-crd: enable rtc")

On SA8540P-ride, the register bank SDAM6 of the first PMIC is not
writable. Following recommendations provided during the review, use
SDAM2 from the second PMIC at offset 0xa0 instead.
Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
Signed-off-by: Eric Chanudet <echanude@redhat.com>
Link: https://lore.kernel.org/r/20230809203506.1833205-1-echanude@redhat.comSigned-off-by: Bjorn Andersson <andersson@kernel.org>

arch/arm64/boot/dts/qcom/sa8540p-pmics.dtsi

@@ -14,7 +14,7 @@ pmm8540a: pmic@0 {
 		#address-cells = <1>;
 		#size-cells = <0>;
 
-		rtc@6000 {
+		pmm8540a_rtc: rtc@6000 {
 			compatible = "qcom,pm8941-rtc";
 			reg = <0x6000>, <0x6100>;
 			reg-names = "rtc", "alarm";
@@ -39,6 +39,15 @@ pmm8540c: pmic@4 {
 		#address-cells = <1>;
 		#size-cells = <0>;
 
+		pmm8540c_sdam_2: nvram@b110 {
+			compatible = "qcom,spmi-sdam";
+			reg = <0xb110>;
+			#address-cells = <1>;
+			#size-cells = <1>;
+			ranges = <0 0xb110 0xb0>;
+			status = "disabled";
+		};
+
 		pmm8540c_gpios: gpio@c000 {
 			compatible = "qcom,pm8150-gpio", "qcom,spmi-gpio";
 			reg = <0xc000>;

arch/arm64/boot/dts/qcom/sa8540p-ride.dts

@@ -407,6 +407,21 @@ &pcie3a_phy {
 	status = "okay";
 };
 
+&pmm8540a_rtc {
+	nvmem-cells = <&rtc_offset>;
+	nvmem-cell-names = "offset";
+
+	status = "okay";
+};
+
+&pmm8540c_sdam_2 {
+	status = "okay";
+
+	rtc_offset: rtc-offset@a0 {
+		reg = <0xa0 0x4>;
+	};
+};
+
 &qup0 {
 	status = "okay";
 };