hw/arm/mps2: Configure the AN500 CPU with 16 MPU regions

hw/arm/mps2: Configure the AN500 CPU with 16 MPU regions

The AN500 application note documents that it configures the Cortex-M7
CPU to have 16 MPU regions. We weren't doing this in our emulation,
so the CPU had only the default 8 MPU regions. Set the mpu-ns-regions
property to 16 for this board.

This bug doesn't affect any of the other board types we model in
this source file, because they all use either the Cortex-M3 or
Cortex-M4. Those CPUs do not have an RTL configurable number of
MPU regions, and always provide 8 regions if the MPU is built in.

Cc: qemu-stable@nongnu.org
Reported-by: Corentin GENDRE <cocotroupe20@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20250605141801.1083266-1-peter.maydell@linaro.org