usb: gadget: omap_udc: Remove omap2 support

There are no active users of this code for omap2 as
the boards in use have either TUSB or MUSB controller.

While at it, also fix warnings related to uninitialized
dc_clk and hhc_clk.

Cc: linux-usb@vger.kernel.org
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>