ARM: dts: exynos: Move fixed clocks under root node in Exynos3250

The fixed clocks are kept under dedicated node fixed-rate-clocks, thus a
fake "reg" was added.  This is not correct with dtschema as fixed-clock
binding does not have a "reg" property:

  arch/arm/boot/dts/exynos3250-artik5-eval.dt.yaml: clock@0: 'reg' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20200829172532.29358-2-krzk@kernel.org