Disable CM5's RTC to make Yellow's on-board RTC the primary one (#3706)

With both RTCs enabled, the rpi_rtc is probed as the first one, making the
on-board RTC unused by default. Since the CM5's RTC peripheral can't be used on
Yellow, as the VBAT pin is not connected, disable it completely to fix RTC.

(cherry picked from commit 9d643edb54d25be404b1cc92b64f1eddf48e6a86)

buildroot-external/board/raspberrypi/yellow/patches/linux/0020-ARM-dts-bcm2712-yellow-Disable-rpi_rtc.patch

@@ -0,0 +1,32 @@
+From fc558b6a7174e2b5c37936dd8c13ca29e9273472 Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Jan=20=C4=8Cerm=C3=A1k?= <sairon@sairon.cz>
+Date: Mon, 2 Dec 2024 17:22:04 +0100
+Subject: [PATCH] ARM: dts: bcm2712: yellow: Disable rpi_rtc
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+Yellow has its own I2C RTC and there is no way to power the new peripheral on
+CM5.
+
+Signed-off-by: Jan Čermák <sairon@sairon.cz>
+---
+ arch/arm64/boot/dts/broadcom/bcm2712-rpi-cm5-ha-yellow.dts | 5 +++++
+ 1 file changed, 5 insertions(+)
+
+diff --git a/arch/arm64/boot/dts/broadcom/bcm2712-rpi-cm5-ha-yellow.dts b/arch/arm64/boot/dts/broadcom/bcm2712-rpi-cm5-ha-yellow.dts
+index 469d0fdc971a8..ebdeef5f89881 100644
+--- a/arch/arm64/boot/dts/broadcom/bcm2712-rpi-cm5-ha-yellow.dts
++++ b/arch/arm64/boot/dts/broadcom/bcm2712-rpi-cm5-ha-yellow.dts
+@@ -927,6 +927,11 @@ &i2c1 {
+ 	clock-frequency = <100000>;
+ };
+ 
++/* No way to power CM5's VBAT pin on Yellow, disable its RTC. */
++&rpi_rtc {
++	status = "disabled";
++};
++
+ /* Board-level I2C (RTC and I2S audio) */
+ &rp1_i2c3 {
+ 	pinctrl-names = "default";