ARM: vexpress/ca9: Add unified-cache property to l2 cache node

Commit d9d1f3e2 ("ARM: l2c: check that DT files specify the required
"cache-unified" property") mandates to specify this required property.
Without this property, we get this boot warning:

"L2C: device tree omits to specify unified cache"

This patch adds "cache-unified" property to L2 cache node in vexpress
CA9 device tree.

Signed-off-by: Robert Schwebel <r.schwebel@pengutronix.de>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>