ARM: dts: exynos: Use S5M8767 clock in S3C RTC in Exynos5250 Spring

Use the 32 kHz clock from S5M8767 PMIC in the S3C RTC node. Except
making the S3C RTC working, this also fixes dtbs_check warnings:

  arch/arm/boot/dts/exynos5250-spring.dt.yaml: rtc@101e0000: clocks: [[2, 337]] is too short
  arch/arm/boot/dts/exynos5250-spring.dt.yaml: rtc@101e0000: clock-names: ['rtc'] is too short

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200901075417.22481-10-krzk@kernel.org