mgmt: correct MPU_ALLOW_FLASH_WRITE selection

Select MPU_ALLOW_FLASH_WRITE when ARM_MPU is enabled, not CPU_HAS_MPU.
MPU_ALLOW_FLASH_WRITE is only defined as a symbol when ARM_MPU is
enabled. ARM_MPU is only defined when CPU_HAS_MPU is defined, so the
CPU_HAS_MPU dependency can be dropped.

This fixes a build error when MCUMGR_CMD_IMG_MGMT is enabled but
ARM_MPU is not.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>