usb: gadget: at91_udc: add usb_clk for transition to common clk framework

The AT91 PMC (Power Management Controller) provides an USB clock used by
USB Full Speed host (ohci) and USB Full Speed device (udc).
The usb drivers (ohci and udc) must configure this clock to 48Mhz.
This configuration was formely done in mach-at91/clock.c, but this
implementation will be removed when moving to common clk framework.

This patch adds support for usb clock retrieval and configuration, and is
backward compatible with the current at91 clk implementation (if usb clk
is not found, it does not configure/enable it).

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>