arm: marvell: Move arch-kirkwood/spi.h to arch-mvebu/spi.h

This move makes it possible to use this kirkwood SPI driver from other
MVEBU platforms as well. This will be used by the upcoming Armada XP
support.

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