Use the 32 kHz clock from S2MPS11 PMIC in the S3C RTC node. Except
making the S3C RTC working, this also fixes dtbs_check warnings:
  arch/arm/boot/dts/exynos5420-smdk5420.dt.yaml: rtc@
101e0000: clocks: [[2, 317]] is too short
  arch/arm/boot/dts/exynos5420-smdk5420.dt.yaml: rtc@
101e0000: clock-names: ['rtc'] is too short
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200901075417.22481-14-krzk@kernel.org
 /dts-v1/;
 #include "exynos5420.dtsi"
 #include "exynos5420-cpus.dtsi"
+#include <dt-bindings/clock/samsung,s2mps11.h>
 #include <dt-bindings/gpio/gpio.h>
 
 / {
 
 &rtc {
        status = "okay";
+       clocks = <&clock CLK_RTC>, <&s2mps11_osc S2MPS11_CLK_AP>;
+       clock-names = "rtc", "rtc_src";
 };
 
 &usbdrd_phy0 {