samples: mpu_test: Add Allow Flash Write config

This patch adds a configuration file that tests Flash writes with MPU
enabled.

[david.brown: Put options in prj.conf with comments as per review
 feedback]

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