PM / clk: Add support for adding a specific clock from device-tree

Some drivers using the PM clocks framework need to add specific clocks
from device-tree using a name by calling the functions
of_clk_get_by_name() and then pm_clk_add_clk(). Rather than having
drivers call both functions, add a helper function of_pm_clk_add_clk()
that will call these functions so drivers can call a single function
to add a specific clock from device-tree.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>