arm/km: fix frequency of the SPI NOR Flash

According to our last HW measures, this could be raised while still
compatible with the potential delays on the lines.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Holger Brunck <holger.brunck@keymile.com>
Acked-By: Prafulla Wadaskar <prafulla@marvell.com>