i2c: Add support for designware i2c controller

Earlier, a driver exists in the u-boot source for designware i2c interface. That
driver was specific to spear platforms. This patch implements the i2c controller
as a generic driver which can be used by multiple platforms

The driver files are now renamed to designware_i2c.c and designware_i2c.h and
these are moved into drivers/i2c folder for reusability by other
platforms

Signed-off-by: Vipin Kumar <vipin.kumar@st.com>
Signed-off-by: Amit Virdi <amit.virdi@st.com>