Allow CPUs to be missing in the Arm PMU driver

To support running on some emulators we allow CPU start to fail if the
CPU we are starting is not present. The PMU driver will then check if
the CPU is present and fail to attach if any are missing.

To allow the PMU to be used in such an environment don't fail to attach
when a CPU is missing.

Sponsored by: Arm Ltd

sys/arm/arm/pmu_acpi.c

@@ -82,7 +82,6 @@ madt_handler(ACPI_SUBTABLE_HEADER *entry, void *arg)
 		/* pcpu not found. */
 		device_printf(sc->dev, "MADT: could not find pcpu, "
 		    "ArmMpidr %lx\n", intr->ArmMpidr);
-		ctx->error = ENODEV;
 		return;
 	}