USB: musb: invert arch depend string

The MUSB code relies on platform implementations that currently only
exists for Arm and Blackfin processors, so have the MUSB Kconfig depend
upon those arches.

This should prevent other arches from building MUSB via randconfig.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>