soc: k64f MPU configured to always allow ENET

Because NXP MPU's regions are dynamically enabled/disabled, ENET
device's access maybe restricted when switching out of a task.
Background DMA transfers from RAM to MAC/PHY may happen during MPU
region reconfiguration or core idling.
Enabled ENET (Kinetis MPU Master 3) to always have access to RAM address
space.

Signed-off-by: Andrei Gansari <andrei.gansari@nxp.com>