soc: renesas: Add missing check for non-zero product register address

If the DTB for a device with an RZ/A2 SoC lacks a device node for the
BSID register, the ID validation code falls back to using a register at
address 0x0, which leads to undefined behavior (e.g. reading back a
random value).

This could be fixed by letting fam_rza2.reg point to the actual BSID
register.  However, the hardcoded fallbacks were meant for backwards
compatibility with old DTBs only, not for new SoCs.  Hence fix this by
validating renesas_family.reg before using it.

Fixes: 175f435f ("soc: renesas: identify RZ/A2")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191016143306.28995-1-geert+renesas@glider.be

drivers/soc/renesas/renesas-soc.c

@@ -334,7 +334,7 @@ static int __init renesas_soc_init(void)
 	if (np) {
 		chipid = of_iomap(np, 0);
 		of_node_put(np);
-	} else if (soc->id) {
+	} else if (soc->id && family->reg) {
 		chipid = ioremap(family->reg, 4);
 	}
 	if (chipid) {