i2c: add binding to mark a bus as supporting SMBus-Alert

Since SMBus-Alert is an optional feature of SMBUS which
requires an additional pin, the smbus binding cannot be
used to indicate its support.

Add an additional smbus-alert binding specific for it and
update the description text of smbus to avoid mentioning
SMBus-Alert

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>