ARM: 8309/1: l2c: enforce use of cache-level property

Make sure that we can read the "cache-level" property from the L2 cache
controller node, and ensure its value is 2.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>