Adapt the periph ADC implementation to use vendor defines where
ever possible. Remove duplicate or obsolete defines and adapt
board configuration as required.
Rework SPI periph driver to use proper RIOT GPIO API functions.
Also cleanup header files by using vendor defines and remove
obsolete code. Further, adapt board config accordingly.
The SPI bus frequency/clock is calculated relative to the MCUs
core clock. Currently all boards use the default 32MHz, hence
prescaler settings for SPI are all the same. This PR moves the
default config for 32MHz to the CPU and allows to be overriden
by board config if needed.
