ARM: dts: exynos: Correct S3C RTC bindings in SMDK5410

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