i2c: Emulate SMBus block read over I2C

Let the I2C bus drivers emulate the SMBus Block Read and Block Process
Call transactions if they wish. This requires to define a new message
flag, which i2c-core will use to let the underlying I2C bus driver
know that the first received byte will specify the length of the read
message.

Signed-off-by: Jean Delvare <khali@linux-fr.org>