usb: musb: Add support for ti81xx platform

TI81XX platform has two musb interfaces and uses CPPI4.1 DMA engine.
It has builtin USB PHYs as AM35x. The current set of patches adds support
for one instance and only in PIO mode.

[ balbi@ti.com : make it compile and solve a "may be used
	uninitialized" warning ]

Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Ravi Babu <ravibabu@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>