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