cpufreq: add suspend/resume support in Armada 37xx DVFS driver

Add suspend/resume hooks in Armada 37xx DVFS driver to handle S2RAM
operations. As there is currently no 'driver' structure, create one
to store both the regmap and the register values during suspend
operation.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>