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.