spi: kirkwood: prevent configuring speed exceeding max controller freq

This patch adds a limitation in the kirkwood_spi driver
set_speed hook, which prevents setting too high transfer
speed.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Kostya Porotchkin <kostap@marvell.com>
Tested-by: Kostya Porotchkin <kostap@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>