clk: scpi: don't add cpufreq device if the scpi dvfs node is disabled

Currently we add the virtual cpufreq device unconditionally even when
the SCPI DVFS clock provider node is disabled. This will cause cpufreq
driver to throw errors when it gets initailised on boot/modprobe and
also when the CPUs are hot-plugged back in.

This patch fixes the issue by adding the virtual cpufreq device only if
the SCPI DVFS clock provider is available and registered.

Fixes: 9490f01e ("clk: scpi: add support for cpufreq virtual device")
Reported-by: Michał Zegan <webczat_200@poczta.onet.pl>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Michał Zegan <webczat_200@poczta.onet.pl>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>