usb: musb: dsps: reduce musb instance to one

Currently multiple phy's of the same type are not supported, hence
reduce musb instances to one. This helps in supporting at least one
instance of musb, rather than having none. Even without this, it was
observed that both instances were working (by luck), but this holds
good iff wrapper is part of Image. And it is not correct for both
controller's to be associated with same phy, here it was working
because phy is a nop one. And having wrapper as a module and
rmmod'ing would crash.

This can be reverted once multi phy support for same type is available
and driver is enhanced to make use of it.

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