USB: fsl_usb2_udc: fix broken Kconfig

The patch fixes broken Kconfig caused by the name change of MPC834x option.
It also makes fsl_usb2_udc selectable on new platforms like MPC837x.

Signed-off-by: Li Yang <leoli@freescale.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>