ARM: dts: exynos: Use S2MPS11 clock in S3C RTC in SMDK5420

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