arm64: enable mpu background region

Enable SCTLR_EL1.BR bit to activate MPU background region support. This simplifies MPU
configuration by providing a default MAIR attribute for unmapped regions, reducing the
need for exhaustive region coverage and improving MPU flexibility.

Signed-off-by: hujun5 <hujun5@xiaomi.com>

arch/arm64/src/common/arm64_arch.h

@@ -97,6 +97,7 @@
 #define SCTLR_C_BIT         BIT(2)
 #define SCTLR_SA_BIT        BIT(3)
 #define SCTLR_I_BIT         BIT(12)
+#define SCTLR_BR_BIT        BIT(17)
 
 /* Controls the impact of tag check faults
  * due to loads and stores in EL0 EL1.

arch/arm64/src/common/arm64_mpu.c

@@ -222,6 +222,7 @@ void arm64_mpu_enable(void)
 #ifndef CONFIG_ARM64_DCACHE_DISABLE
           | SCTLR_C_BIT
 #endif
+          | SCTLR_BR_BIT             /* Background region enable */
          );
   write_sysreg(val, sctlr_el1);
   UP_MB();