arm64: dts: qcom: sm6115: Provide xo clk to rpmcc

rpmcc used to rely on global clock lookup (and still does so for
backwards compat reasons) of "xo_board", which was common back
when we did not care about things like underscores in node names.
Nowadays it expects to be fed a reference to the fixed clock.
Satisfy that requirement to make sure rpm clock rates are not all
stuck at zero.

Fixes: 97e563bf ("arm64: dts: qcom: sm6115: Add basic soc dtsi")
Reported-by: Adam Skladowski <a39.skl@gmail.com>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221208201401.530555-2-konrad.dybcio@linaro.org