arm: am57xx: cl-som-am57x: invoke clock API to enable/disable clocks

arm: am57xx: cl-som-am57x: invoke clock API to enable/disable clocks

Invoke enable_usb_clocks during board_usb_init and disable_usb_clocks
during board_usb_exit to enable and disable clocks respectively.

Modifications:
* Enable USB clocks in the OMAP version of the function
  board_usb_init.
* Disable USB clocks in the OMAP version of the function
  board_usb_cleanup.

Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Uri Mashiach <uri.mashiach@compulab.co.il>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>