i2c: Implement consistent i2c no msgs behaviour

The I2C drivers handle an empty list of I2C messages inconsistenty.
There are two different behaviours, one set of drivers dectects a
requests to transfer zero messages and return -EINVAL while the other
group simple transfer no data and return success.

Adopt the latter behaviour consistently across all drivers.  Update
the i2c.h API documentation to reflect this behaviour.

Change-Id: I427fc1b0e18ddc04b7b59c294e0240b3d6ca4073
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>