ARM: orion: Move cache-feroceon-l2.h out of plat-orion

With the gradual move to DT, kirkwood has become a lot less dependent
on plat-orion. cache-feroceon-l2.h is the last dependency. Move it out
so we can drop plat-orion when building DT only kirkwood boards.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>