ARM: dts: s5pv210: add RTC 32 KHz clock in Goni

The S3C RTC requires 32768 Hz clock as input which is provided by PMIC.
However there is no such clock provider but rather a regulator driver
which registers the clock as a regulator.  This is an old driver which
will not be updated so add a workaround - a fixed-clock to fill missing
clock phandle reference in S3C RTC.

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/20200907161141.31034-13-krzk@kernel.org