usb: musb: make all glue layer export struct musb_platform_ops

preparing to a big refactor on musb code. We need
to be able to compile in all glue layers (or at
least all ARM-based ones) together and have a
working binary.

While preparing for that, we move every glue
layer to export only one symbol, which is
a struct musb_platform_ops, and make all
other functions static.

Later patches will come to allow for compiling
all glue layers together and have a working
binary.

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