cpufreq: qoriq: convert to a platform driver

The driver has to be manually loaded if it is built as a module. It
is neither exporting MODULE_DEVICE_TABLE nor MODULE_ALIAS. Moreover,
no platform-device is created (and thus no uevent is sent) for the
clockgen nodes it depends on.

Convert the module to a platform driver with its own alias. Moreover,
drop whitelisted SOCs. Platform device will be created only for the
compatible platforms.

Reviewed-by: Yuantian Tang <andy.tang@nxp.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Mian Yousaf Kaukab <ykaukab@suse.de>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>