ARM: Kirkwood: fix disabling CACHE_FEROCEON_L2

Move the CACHE_FEROCEON_L2 test to kirkwood_l2_init, since linking
fails on the reference to feroceon_l2_init.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>