irqchip/gic-pm: Update driver to use of_pm_clk_add_clk

Commit 498b5fdd ("PM / clk: Add support for adding a specific clock
from device-tree") add a new helper function for adding a clock from
device-tree to a device. Update the GIC-PM driver to use this new
function to simplify the driver.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>