usb: musb: debugfs: cope with blackfin's oddities

Blackfin's MUSB implementation lacks a bunch of
registers which they end up not defining a macro
for. In order to avoid build breaks, let's ifdef
out some of the registers from our regdump debugfs
utility so that we don't try to use those on
Blackfin builds.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>