arm: soc: nxp k6x: mpu: add Bus Master 3 User Mode access bits

Ethernet on K64F is connected via Logical Bus Master 3.
Section 19.3.8 of K64F reference manual establishes bits 20-18
(M3UM) on page 427 as "Bus Master 3 User Mode Access Control".

To fix RWX user mode access via Bus Master 3 when MPU is enabled,
we need to add these bits to the MPU region descriptors.

This fixes ETH0 on K64F when MPU is enabled.

Fix recommended by Maureen Helm <maureen.helm@nxp.com>

Signed-off-by: Michael Scott <michael.scott@linaro.org>