arm64: dts: qcom: sc8280xp-x13s: enable rtc

The Lenovo X13s firmware does not implement the UEFI time runtime
services so the RTC in the PM8280K PMIC needs to be accessed directly.

To complicate things further, the RTC control and time registers are
read-only on this platform so an offset must be stored in some other
machine-specific non-volatile memory which an RTC driver can take into
account when reading or updating the time.

The UEFI firmware (and Windows) use a UEFI variable for this:

	882f8c2b-9646-435f-8de5-f208ff80c1bd-RTCInfo

but the offset can only be accessed via the Qualcomm UEFI Secure
Application residing in the TEE as the firmware does not implement the
variable runtime services either.

While it is possible to access this UEFI variable from Linux on the
X13s, this requires using a fairly complex and reverse-engineered
firmware interface. As the only benefit of doing so is to make sure that
the UEFI (Windows) and Linux time never gets out of sync, it seems
preferable to use the PMIC scratch registers for storing an offset
instead. This also avoids flash wear in case of RTC drift, etc.

So instead of using the UEFI RTC offset, reserve four bytes in one of
the PMIC SDAM scratch-register blocks to hold the RTC offset.
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230202155448.6715-23-johan+linaro@kernel.org

arch/arm64/boot/dts/qcom/sc8280xp-lenovo-thinkpad-x13s.dts

@@ -766,6 +766,21 @@ &pmk8280_pon_resin {
 	status = "okay";
 };
 
+&pmk8280_rtc {
+	nvmem-cells = <&rtc_offset>;
+	nvmem-cell-names = "offset";
+
+	status = "okay";
+};
+
+&pmk8280_sdam_6 {
+	status = "okay";
+
+	rtc_offset: rtc-offset@bc {
+		reg = <0xbc 0x4>;
+	};
+};
+
 &pmk8280_vadc {
 	status = "okay";