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