clk: qcom: gcc-ipq6018: mark gcc_xo_clk_src as critical

The XO clock source is always-on in hardware and cannot be gated.
Without CLK_IS_CRITICAL, runtime PM of downstream consumers (such as
the CMN PLL driver) cascades a disable up to gcc_xo_clk_src, causing
a branch status timeout warning.

The IPQ8074 GCC driver already marks this clock as CLK_IS_CRITICAL.
Apply the same fix to IPQ6018.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260311182147.30266-1-ansuelsmth@gmail.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>