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 dtbs_check warnings:

  rtc@e2800000: clocks: [[2, 145]] is too short
  rtc@e2800000: clock-names: ['rtc'] is too short
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200907161141.31034-15-krzk@kernel.org