st_smi: Add support for SPEAr SMI driver

st_smi: Add support for SPEAr SMI driver

SMI is the serial memory interface controller provided by ST.

Earlier, a driver exists in the u-boot source code for the SMI IP. However, it
was specific to spear platforms. This commit converts the same driver to a more
generic driver. As a result, the driver files are renamed to st_smi.c and
st_smi.h and moved into drivers/mtd folder for reusability by other platforms
using smi controller peripheral.

Signed-off-by: Vipin Kumar <vipin.kumar@st.com>
Signed-off-by: Amit Virdi <amit.virdi@st.com>
Signed-off-by: Stefan Roese <sr@denx.de>