wb_i2c_bridge: Make FSM watchdog setup more generic

This is done by adding a generic to the entity, which is connected directly to
the gc_fsm_watchdog component instantiated within the wb_i2c_bridge. The user
should calculate the appropriate watchdog timeout value and set it via this
generic.

The instantiation template in the wishbone_pkg is also updated.

Signed-off-by: Theodor Stana <t.stana@cern.ch>