cpufreq: qcom-cpufreq-hw: Avoid stack buffer for IRQ name

Registering an IRQ requires the string buffer containing the name to
remain allocated, as the name is not copied into another buffer.

So let's add a irq_name field to the data struct instead, which is
guaranteed to have the appropriate lifetime.

Cc: Thara Gopinath <thara.gopinath@linaro.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Andy Gross <agross@kernel.org>
Cc: linux-arm-msm@vger.kernel.org
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>