ARM: dts: am335x-icev2: Add system-power-controller to RTC node

On the am335x-icev2 board [1], AM3359 PMIC_PWR_EN (ZCZ ball C6) is
connected to PWRHOLD (pin 1) on the TPS65910A3 PMIC. The addition of
system-power-controller to the rtc node will enable the capability of
the RTC driver to poweroff the board.

Based on commit 15c7be47fb7c ("ARM: dts: am335x: Add rtc node as
system-power-controller") by Keerthy in ti-linux-5.4.y branch [2].

[1] https://www.ti.com/tool/TMDSICE3359
[2] git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git

Cc: Dave Gerlach <d-gerlach@ti.com>
Cc: Keerthy <j-keerthy@ti.com>
Link: https://lore.kernel.org/linux-omap/20211012191311.879838-1-dfustini@baylibre.com/Suggested-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/boot/dts/am335x-icev2.dts

@@ -512,3 +512,7 @@ ethphy1: ethernet-phy@3 {
 &pruss_tm {
 	status = "okay";
 };
+
+&rtc {
+	system-power-controller;
+};