arm64: dts: rockchip: Add 32k clk on rk3399-gru

This adds the 32k clock to the RK3399 Gru board file, which is provided
by a Silego oscillator on Gru boards.

Even though it's not directly used, muxes will end up traversing the
entire clk tree on calls to determine_rate if it doesn't exist. This
is because the 32k clk is listed as a possible parent on some clks.
Since the clk doesn't know about the 32k clk (it was never registered),
it triggers a global search for it. This can happen about 40 times per
second, which isn't great for power.
Signed-off-by: Derek Basehore <dbasehore@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
[moved clock position and adapted commit message a bit]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3399-gru.dtsi

@@ -262,6 +262,13 @@ pp3300_trackpad: pp3300-trackpad {
 	pp5000_usb_a_vbus: pp5000 {
 	};
 
+	ap_rtc_clk: ap-rtc-clk {
+		compatible = "fixed-clock";
+		clock-frequency = <32768>;
+		clock-output-names = "xin32k";
+		#clock-cells = <0>;
+	};
+
 	gpio_keys: gpio-keys {
 		compatible = "gpio-keys";
 		pinctrl-names = "default";