spi: kirkwood_spi.c: Add driver model support

spi: kirkwood_spi.c: Add driver model support

This patch adds driver model support to the kirkwood SPI driver. Which
is also used on the MVEBU SoC's, now being converted to DM. Non-DM
support is still available for the "older" platforms using this
driver, like kirkwood.

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>