ARM: AM43xx: Add functions to enable and disable USB clocks

Added functions to enable and disable USB clocks which can be invoked
during USB init and USB exit respectively.

Cc: Roger Quadros <rogerq@ti.com>
Cc: Tero Kristo <t-kristo@ti.com>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>