arm: marvell: Move arch/kirkwood.h to arch/soc.h

This move makes is possible to use this header not only from kirkwood
platforms but from all Marvell mvebu platforms.

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Luka Perkov <luka@openwrt.org>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>