arm: mpu: Remove FLASH_0 mpu region when XIP=n

When not using CONFIG_XIP (CONFIG_XIP=n)
the FLASH_0 mpu region needs to be removed,
otherwise it will have the default base
address = 0, which means that MPU will try
to configure the region with address 0.
We don't want this as in some situations
address 0 can be a restricted memory region
such as ROM code.

Signed-off-by: Alexandru Lastur <alexandru.lastur@nxp.com>