[ARM] Kirkwood: support L2 writeback mode

This patch allows booting Kirkwood with the L2 in writeback mode,
by reading the WT override bit from the L2 config register and
passing that into the Feroceon L2 init routine, instead of assuming
that the WT override bit will always be set

Signed-off-by: Saeed Bishara <saeed@marvell.com>
Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>