PIC32MZ SPI:  Implement exchange() method; update SPI_REGDEBUG configuration and clean up implementation

Adds architecture support for the STM32F372 and F373 (no board support yet). Only tested on STM32F373CC, but should work on the rest.  Contributed by  Marten Svanfeldt.

Rename all use of up_boardinitialize().  Should not use common microprocessing naming convention but rather the microprocessor-specific naming conventioni

arch/: board function prototypes are now in include/nuttx/board.h.  Remove from architecture header file; Add inclusion of nuttx/board.h to all files referencing board functions

PIC32MZ: Fix some repeated typos and work around an issue with passing defined parameters to a macro that takes multiple parameters

PIC32MX/Z: Partial review of PIC32MZ cp0 register -- need to do more; Also found issues with definitions for PIC32MX -- need to be retested

PIC32MZ: Add file for GPIO interrupt support.  There are issues so configuration is EXPERIMENTAL for now