boards: arm: mps2_an521: remove the arm,num-mpu-regions property

The SSE-200 implementation on AN521 only has 8 regions.
Although the AN521 FPGA image implements the documented value,
qemu-system-arm currently shows in MPU_TYPE the incorrect value of 16.

Removing the property definition allows the code to rely on the register
read value and work on both qemu-system-arm & the physical board.

Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>

boards/arm/mps2_an521/mps2_an521.dts

@@ -79,7 +79,6 @@
 			mpu: mpu@e000ed90 {
 				compatible = "arm,armv8m-mpu";
 				reg = <0xe000ed90 0x40>;
-				arm,num-mpu-regions = <16>;
 			};
 		};
 	};