arm64: dts: qcom: msm8998: Provide missing "xo" and "sleep_clk" to GCC

In a future patch the GCC driver will stop requesting this xo clock by
its global "xo" name, in favour of having an explicit phandle here in
the DT.  Aside from that this clock in addition to the mandatory
"sleep_clk" were never passed despite being required by the relevant
dt-bindings.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210911120101.248476-1-marijn.suijten@somainline.org