PCI: dra7xx: Add shutdown handler to cleanly turn off clocks

Add shutdown handler to cleanly turn off clocks.  This will help in cases of
kexec where in a new kernel can boot abruptly.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>