arm64: dts: qcom: sc8280xp: add missing 300MHz

When booting a Thinkpad x13s, we see the message

[    0.997647] cpu cpu0: failed to update OPP for freq=300000

So, lets add in 300MHz to make it happy

Signed-off-by: Steev Klimaszewski <steev@kali.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220718225714.8074-1-steev@kali.org