arm: core: mpu: Add Allow write on Flash

This patch adds the allow flash write CONFIG option to the ARM MPU
configuration in privileged mode.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>
Signed-off-by: Michael Scott <michael.scott@linaro.org>
Signed-off-by: David Brown <david.brown@linaro.org>