arm: allow the debug stuff in CP14 to be disabled at compile time

The upcoming QCA ipq401x support detects the CP14 debug features,
but any attempt to use it causes an undefined instruction error.
It apparently needs a specific TZ image loaded by the early bootloader
(SBL) in order to enable these kinds of features.

So add a new kernel option that explicitly disables this in the
arm code - the debugger works fine without it.

sys/arm/arm/debug_monitor.c

@@ -960,6 +960,10 @@ dbg_reset_state(void)
 void
 dbg_monitor_init(void)
 {
+#ifdef	ARM_FORCE_DBG_MONITOR_DISABLE
+	db_printf("ARM Debug Architecture disabled in kernel compilation.\n");
+	return;
+#else
 	int err;
 
 	/* Fetch ARM Debug Architecture model */
@@ -1001,6 +1005,7 @@ dbg_monitor_init(void)
 
 	db_printf("HW Breakpoints/Watchpoints not enabled on CPU%d\n",
 	    PCPU_GET(cpuid));
+#endif	/* ARM_FORCE_DBG_MONITOR_DISABLE */
 }
 
 CTASSERT(sizeof(struct dbreg) == sizeof(((struct pcpu *)NULL)->pc_dbreg));

sys/conf/options.arm

@@ -1,6 +1,7 @@
 #$FreeBSD$
 ARMV6			opt_global.h
 ARMV7			opt_global.h
+ARM_FORCE_DBG_MONITOR_DISABLE	opt_ddb.h
 CPSW_ETHERSWITCH	opt_cpsw.h
 CPU_ARM1176		opt_global.h
 CPU_CORTEXA		opt_global.h