On system suspend, the runtime_suspend() driver hook doesn't get
called for USB phy and so the clocks are not disabled in the driver.
This causes the L3INIT_960M_GFCLK and L3INIT_480M_GFCLK to remain
active on the DRA7 platform while in system suspend.

In case of pcie-phy, the runtime_suspend hook gets called after
the suspend hook so we introduce a flag phy->enabled to keep
track if our clocks are enabled or not to prevent multiple
enable/disables.

Add suspend/resume hooks to the driver.
Move enabling/disabling clock code into helper functions.
Reported-by: Nishant Menon <nm@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>