usb: musb: ux500: attempt to find channels by name before using pdata

If we can ever get to a state where we can solely search for DMA channels
by name, this will almost completely alleviate the requirement to pass
copious amounts of information though platform data. Here we take the
first step towards this. The next step will be to enable Device Tree
complete with name<->event_line mapping.

Cc: linux-usb@vger.kernel.org
Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>