i2c: bus: added single message xfer functions.

Added the API functions:

i2c_master_send:
	Send a single I2C message of a given length to an I2C client.

i2c_master_rcv:
	Receive a single I2C message of a given length from an I2C
	client.

Signed-off-by: Michel Megens <dev@michelmegens.net>