Blackfin: overhaul i2c driver

The current Blackfin i2c driver does not work properly with certain devices
due to it breaking up transfers incorrectly.  This is a rewrite of the
driver and relocates it to the newer place in the source tree.

Also remove duplicated I2C speed defines in Blackfin board configs and
disable I2C slave address usage since it isn't implemented.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>