spi: kirkwood_spi.c: Prepare for driver model support

This patch prepares the Kirkwood SPI driver, also used on the MVEBU board
(Armada XP / 38x), for the conversion to driver model.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Valentin Longchamp <valentin.longchamp@keymile.com>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Jagan Teki <jteki@openedev.com>
Cc: Simon Glass <sjg@chromium.org>