cpufreq: qcom-hw: Use optional irq API

Use platform_get_irq_optional() to avoid a noisy error message when the
irq isn't specified. The irq is definitely optional given that we only
care about errors that are -EPROBE_DEFER here.

Cc: Thara Gopinath <thara.gopinath@linaro.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>