PM / Domains: Fix potential NULL pointer dereference

In the function of_genpd_get_from_provider(), we never check to see if
the argument 'genpdspec' is NULL before dereferencing it. Add error
checking to handle any NULL pointers.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>