disas/microblaze: Fix warnings caused by missing 'static' attribute

Warnings from the Sparse static analysis tool:

disas/microblaze.c:289:3: warning:
 symbol 'opcodes' was not declared. Should it be static?
disas/microblaze.c:570:6: warning:
 symbol 'register_prefix' was not declared. Should it be static?
disas/microblaze.c:571:6: warning:
 symbol 'special_register_prefix' was not declared. Should it be static?
disas/microblaze.c:572:6: warning:
 symbol 'fsl_register_prefix' was not declared. Should it be static?
disas/microblaze.c:573:6: warning:
 symbol 'pvr_register_prefix' was not declared. Should it be static?

Remove the unused variable special_register_prefix.
The variable pvr_register_prefix was unused, too, but can be used.
Add also 'const' where possible.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>