The S3C RTC requires 32768 Hz clock as input which is provided by PMIC
(Maxim MAX8997).  However there is no clock provided for the PMIC and
the driver registers the clock as regulator.  This is an old driver
which will not be updated so add a workaround:
1. Enable the "clock" regulator in PMIC,
2. Add a fixed-clock to fill missing clock phandle reference in S3C RTC.

This allows to enable the S3C RTC and fixes dtbs_check warnings:

  arch/arm/boot/dts/exynos4210-origen.dt.yaml: rtc@10070000: clocks: [[5, 346]] is too short
  arch/arm/boot/dts/exynos4210-origen.dt.yaml: rtc@10070000: clock-names: ['rtc'] is too short
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20200830135200.24304-4-krzk@kernel.org