soc: include: fvp_aemv8r: Define device memory as device tree node

A recent change introduced the possibility to declare MPU memory
regions using the device tree and the framework described in
zephyr/linker/devicetree_regions.h.

So remove the device region declared in mpu_regions[] and the used
defines for the addresses, rename REGION_DEVICE_ATTR to REGION_IO_ATTR
in arm_mpu.h to be compatible with the framework and add a device tree
node to fvp_baser_aemv8r.dts to describe the device memory region.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>