armv7: Fix BeagleBone Black panic on system start

There is now assertion which requires all memory allocations of positive size.
Negative and zero-sized allocations lead to panic, so plug them off.

Reviewed by:	imp, emaste
Differential Revision:	https://reviews.freebsd.org/D39846

sys/arm/ti/ti_sysc.c

@@ -306,6 +306,9 @@ parse_regfields(struct ti_sysc_softc *sc) {
 
 	/* Grab the content of reg properties */
 	nreg = OF_getproplen(node, "reg");
+	if (nreg <= 0)
+		return (ENXIO);
+
 	reg = malloc(nreg, M_DEVBUF, M_WAITOK);
 	OF_getencprop(node, "reg", reg, nreg);