samples: nvs: Allow mpu flash write
samples: nvs: Allow mpu flash write

Make this sample work on mpu-enabled platforms by configuring the mpu to
allow flash writes.

Signed-off-by: Maureen Helm <87a696db5547fa477bda9f9e0f00013c839c14d3@nxp.com>

zephyrproject-rtos/zephyr,ldts/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,explora26/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,punitvara/zephyr,Vudentz/zephyr,kraj/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,ldts/zephyr,punitvara/zephyr,kraj/zephyr,finikorg/zephyr,nashif/zephyr,kraj/zephyr,punitvara/zephyr,kraj/zephyr,finikorg/zephyr,punitvara/zephyr,explora26/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,punitvara/zephyr,Vudentz/zephyr,nashif/zephyr,explora26/zephyr,nashif/zephyr