cpufreq: vexpress-spc: use macros instead of hardcoded values for cluster ids

A15 and A7 cluster identifiers are fixed to 0 and 1 respectively. There are
macros for the same and used in most of the places except this instance.

Lets use macros instead of hardcoded values for cluster ids even here.

Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>