clk: scpi: add support for cpufreq virtual device

The clocks for the CPUs are provided by SCP and are managed by this
clock driver. So the cpufreq device needs to be added only after the
clock get registered and removed when this driver is unloaded.

This patch manages the cpufreq virtual device based on the clock
availability.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Mike Turquette <mturquette@baylibre.com>
Cc: Liviu Dudau <Liviu.Dudau@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Jon Medhurst (Tixy) <tixy@linaro.org>
Cc: linux-clk@vger.kernel.org