i2c-viapro: Add support for the VT8237A and VT8251

Documentation update included. Compile tested.
Signed-off-by: NRudolf Marek <r.marek@sh.cvut.cz>
Signed-off-by: NJean Delvare <khali@linux-fr.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>