ARM: OMAP: rx51: change musb mode to OTG

Peripheral-only mode got broken in v3.11-rc1 because of unknown reasons.
Change the mode to OTG, in practice that should work equally well even
when/if the regression gets fixed.

Note that the peripheral-only regression is a separate patch, this change
is still correct as the role is handled by hardware.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Olof Johansson <olof@lixom.net>