PCI: dra7xx: Group PHY API invocations

No functional change.  PHY APIs like phy_init()/phy_power_on() are invoked
from multiple places.  Group all the PHY APIs in dra7xx_pcie_enable_phy()
and dra7xx_pcie_disable_phy() and use these functions for enabling or
disabling the PHY.

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