arm64: dts: qcom: sm8250: correct sdhc_2 xo clk

sdhc_2 uses 19200000 Hz clock rather than wrongly specified xo_board
(39400000 Hz). Specify correct clock to fix DLL setup for SDR104 mode.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Fixes: c4cf0300

 ("arm64: dts: qcom: sm8250: Add support for SDC2")
Link: https://lore.kernel.org/r/20210109011252.3436533-1-dmitry.baryshkov@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>