ARM: dts: exynos: drop undocumented samsung,sata-freq property in Exynos5250

The samsung,sata-freq property is not used (and not documented by
generic AHCI platform bindings), so can be safely dropped.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210811083859.28234-2-krzysztof.kozlowski@canonical.com