ARM: 6953/1: DT: don't try to access physical address zero

If the DT physical address is zero, this is equivalent to no DT.
Especially when the actual RAM physical address is not located at zero,
the result of phys_to_virt() would point to la-la-land and crash the
kernel, which crash is completely silent this early during boot.
Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/devtree.c

@@ -76,6 +76,9 @@ struct machine_desc * __init setup_machine_fdt(unsigned int dt_phys)
 	unsigned long dt_root;
 	const char *model;
 
+	if (!dt_phys)
+		return NULL;
+
 	devtree = phys_to_virt(dt_phys);
 
 	/* check device tree validity */