CPUFREQ: Save and restore max and min scaling freq

Modify the cpufreq.c driver to save and restore any user modified
scaling_max_freq and scaling_min_freq when CPU hotplugging. By
default, any cap on maximum scaling frequency is removed when a CPU is
taken offline and then brought back online again and the physical
maximum or minimum frequency is restored instead. This patch changes
that behaviour.

Change-Id: If2faa481006ae0ef9a2a5c7c5699386e6ea6f623