mtd: ST SPEAr: Add SMI driver for serial NOR flash

SPEAr platforms (spear3xx/spear6xx/spear13xx) provide SMI (Serial Memory
Interface) controller to access serial NOR flash. SMI provides a simple
interface for SPI/serial NOR flashes and has certain inbuilt commands
and features to support these flashes easily. It also makes it possible
to map an address range in order to directly access (read/write) the SNOR
over address bus. This patch intends to provide serial nor driver support
for spear platforms which are accessed through SMI.

Signed-off-by: Shiraz Hashim <shiraz.hashim@st.com>
Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
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>