Because NXP MPU's regions are dynamically enabled/disabled, USB
device's access maybe restricted when switching out of a task.
Background DMA transfers to/from RAM may happen during MPU region
reconfiguration or core idling.
Enabled USB (Kinetis MPU Master 4) to always have access to RAM address
space.
Signed-off-by: Andrei Gansari <andrei.gansari@nxp.com>