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 a fixed-clock.  This fixes S3C RTC
and silences dtbs_check warnings like:

  arch/arm/boot/dts/exynos5410-smdk5410.dt.yaml: rtc@101e0000: clocks: [[5, 317]] is too short
  arch/arm/boot/dts/exynos5410-smdk5410.dt.yaml: rtc@101e0000: clock-names: ['rtc'] is too short
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200901075417.22481-5-krzk@kernel.org