mfd: omap-usb-host: cleanup clock management code

All ports have similarly named port clocks so we can
bunch them into a port data structure and use for loop
to enable/disable the clocks.

Dynamically allocate and get clocks based on number of ports
available on the platform

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>