usb: musb: Make busctl_offset an io-op rather then a define

The Allwinner (sunxi) implementation of the musb has its busctl registers
indexed by the MUSB_INDEX register rather then in a flat address space.

This commit turns MUSB_BUSCTL_OFFSET from a macro into an io-op which can
be overridden from the platform ops.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>