tests: flash_map: Enable mpu and allow flash write
tests: flash_map: Enable mpu and allow flash write

It shouldn't be necessary to disable the mpu for this test. Enable
CONFIG_MPU_ALLOW_FLASH_WRITE=y instead.

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

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