I2C: add new command i2c reset.

If I2C Bus is blocked (see doc/I2C_Edge_Conditions),
it is not possible to get out of this, until the
complete Hardware gets a reset. This new commando
calls again i2c_init (and that calls i2c_init_board
if defined), which will deblock the I2C Bus.

Signed-off-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Wolfgang Denk <wd@denx.de>