Upstream pic32 changes

[abhijit-mahajani-imgtec]

Contiki currently has support for PIC32MX795F512L(PIC32 model), which is used by SEED EYE board.
We have extended this(cpu/pic32) module to support PIC32MX470F512H in order to support Mikro-e 6LoWPAN Clicker board.
All these changes need to be upstreamed to contiki-os

[abhijit-mahajani-imgtec]

Upstreaming PR has been raised contiki-os#2116