cc3200/mods/pybi2c: Make machine.I2C constructor/init conform to HW API.

This is a user-facing change to the cc3200's API, to make it conform to the
new machine hardware API.  The changes are:

- change I2C constructor to: I2C(id=0, *, freq=100000, scl=None, sda=None)
- change I2C init to: init(*, freq, scl, sda)
- removal of machine.I2C.MASTER constant
- I2C str/repr no longer prints I2C.MASTER

To update existing code it should be enough to just remove the I2C.MASTER
constant from contructor/init for I2C.