USB: musb: allow the Blackfin vrsel gpio to be active low

Rather than hardcoding the gpio levels for vrsel, allow the platform
resources to handle this so boards can be active high or low.

Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>