mtd: add device-tree support to spear_smi

This patch adds support to configure the SPEAr SMI driver via
device-tree instead of platform_data.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>