ARM: dts: exynos: Correct S3C RTC bindings in SMDK4412

The S3C RTC requires 32768 Hz clock as input which is provided by PMIC.
However the PMIC is not described in DTS at all so at least add a
workaround to model its clock with fixed-clock and silence dtbs_check
warnings like:

  arch/arm/boot/dts/exynos4412-smdk4412.dt.yaml: rtc@10070000: clocks: [[7, 346]] is too short
  arch/arm/boot/dts/exynos4412-smdk4412.dt.yaml: rtc@10070000: clock-names: ['rtc'] is too short

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200830135200.24304-31-krzk@kernel.org