usb: musb: Add function pointers for IO access functions

MUSB currently breaks badly if we try to build in support
for multiple platforms. This also happens if done as loadable
modules, which is not nice for distros.

Let's fix the issue by adding new struct musb_io for the IO
access functions that the platform code can populate. Note
that we don't want to use the current ops as that's really
platform_data and and set as a const.

This should allow eventually adding function pointers also
for the DMA code to struct musb_io, but that's a whole
different set of patches. For now, let's just fix the PIO
access.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>