Add dummy implementation of generic timer cp15 registers

Add a dummy implementation of the cp15 registers for the generic
timer (found in the Cortex-A15), just sufficient for Linux to
decide that it can't use it. This requires at least CNTP_CTL and
CNTFRQ to be implemented as RAZ/WI; we RAZ/WI all of c14.
Reviewed-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target-arm/cpu.h

@@ -382,6 +382,7 @@ enum arm_features {
     ARM_FEATURE_VAPA, /* cp15 VA to PA lookups */
     ARM_FEATURE_ARM_DIV, /* divide supported in ARM encoding */
     ARM_FEATURE_VFP4, /* VFPv4 (implies that NEON is v2) */
+    ARM_FEATURE_GENERIC_TIMER,
 };
 
 static inline int arm_feature(CPUARMState *env, int feature)

target-arm/helper.c

@@ -1764,7 +1764,11 @@ void HELPER(set_cp15)(CPUState *env, uint32_t insn, uint32_t val)
             goto bad_reg;
         }
         break;
-    case 14: /* Reserved.  */
+    case 14: /* Generic timer */
+        if (arm_feature(env, ARM_FEATURE_GENERIC_TIMER)) {
+            /* Dummy implementation: RAZ/WI for all */
+            break;
+        }
         goto bad_reg;
     case 15: /* Implementation specific.  */
         if (arm_feature(env, ARM_FEATURE_XSCALE)) {
@@ -2134,7 +2138,11 @@ uint32_t HELPER(get_cp15)(CPUState *env, uint32_t insn)
         default:
             goto bad_reg;
         }
-    case 14: /* Reserved.  */
+    case 14: /* Generic timer */
+        if (arm_feature(env, ARM_FEATURE_GENERIC_TIMER)) {
+            /* Dummy implementation: RAZ/WI for all */
+            return 0;
+        }
         goto bad_reg;
     case 15: /* Implementation specific.  */
         if (arm_feature(env, ARM_FEATURE_XSCALE)) {