Increase the fdtmemreserv array limit to boot on POWER9

Discussing with others, this needs to be at least 20 to boot on some POWER9
nodes.  Linux made a similar change for the same reason, so increase to 32
to give us some extra breathing room as well.  The input and output arrays
are sized at 256, so much greater than the increase in the property array
size.

sys/powerpc/ofw/ofw_machdep.c

@@ -230,7 +230,7 @@ excise_fdt_reserved(struct mem_region *avail, int asz)
 	struct {
 		uint64_t address;
 		uint64_t size;
-	} fdtmap[16];
+	} fdtmap[32];
 	ssize_t fdtmapsize;
 	phandle_t chosen;
 	int i, j, k;