ARM: dts: exynos: Add max77686 clocks for S3C RTC in SMDK5250

Add clock-cells to max77686 PMIC node so its 32 kHz clocks could be used
later in the S3C RTC node. Except making the S3C RTC working, this also
fixes dtbs_check warnings:

  arch/arm/boot/dts/exynos5250-smdk5250.dt.yaml: rtc@101e0000: clocks: [[2, 337]] is too short
  arch/arm/boot/dts/exynos5250-smdk5250.dt.yaml: rtc@101e0000: clock-names: ['rtc'] is too short

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200901075417.22481-9-krzk@kernel.org