cpufreq: scpi: use new scpi_ops functions to remove duplicate code

scpi_ops now provide APIs to get the transition_latency and to add
OPPs to the devices making those logic redundant here.

This patch makes use of those APIs and removes the redundant code in
this driver.

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