i2c: Blackfin TWI: implement I2C_FUNC_SMBUS_I2C_BLOCK functionality

Some drivers need i2c_smbus_read_i2c_block_data() functionality, so add
support for it to the Blackfin I2C bus driver.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
[ben-linux@fluff.org: shortened subject]
Signed-off-by: Ben Dooks <ben-linux@fluff.org>