[ARM] Kirkwood: add support for L2 cache WB/WT selection

Feroceon L2 cache can work in eighther write through or write back mode
on Kirkwood. Add the option to configure this mode according to Kconfig.

Signed-off-by: Ronen Shitrit <rshitrit@marvell.com>
Signed-off-by: Nicolas Pitre <nico@marvell.com>