bhnd(4): Add missing BCM4312 backplane clock speed entry.

The default 80MHz clock speed returned by bhnd_pmu_si_clock() was already
correct; this just prevents the "No backplane clock specified" warning
printf from being emitted when querying backplane clock speed.

Sponsored by:	The FreeBSD Foundation

sys/dev/bhnd/cores/pmu/bhnd_pmu_subr.c

@@ -2394,6 +2394,7 @@ bhnd_pmu_si_clock(struct bhnd_pmu_query *sc)
 		clock = bhnd_pmu1_cpuclk0(sc);
 		break;
 
+	case BHND_CHIPID_BCM4312:
 	case BHND_CHIPID_BCM4313:
 		/* 80MHz backplane clock */
 		clock = 80000 * 1000;