arm64: dts: exynos: move XTCXO clock frequency to board in Exynos Auto v9

The external oscillator - XTCXO - is an input to the SoC.  It is defined
in the Exynos Auto v9 SoC DTSI, because all boards will provide it and
clock controller bindings expect it, however the actual frequency of the
clock should be determined by the board.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Chanho Park <chanho61.park@samsung.com>
Link: https://lore.kernel.org/r/20220503092631.174713-1-krzysztof.kozlowski@linaro.org