Added basic MX470 support

The MX470 is an awkward chip to support.  It has PPS like the MX1/2, but is wildly different from the other MX4 chips.  So, a new category `__PIC32MX47X__` has had to be made.  On top of that there are differences between the L and H versions (different PPS pins available), so two further sub-categories of `__PIC32MX47XH__` and `__PIC32MX47XL__` have had to be made as well.