usb: musb: introduce struct musb_platform_ops

This will be passed to musb_core by platform glue
layer in order to make it easier to compile support
for several HW glue layers.

Later patches will come using this structure and
also moving HW glue layers to its own platform
driver; the idea is to be able to handle platform
peculiarities in a manner which doesn't affect one
another.

Signed-off-by: Felipe Balbi <balbi@ti.com>